Speculation: On the Foretellers' Master, the Realm of Sleep, and Xehanort's Master Plan

[The_Echo]

So this is by no means a solid theory. Evidence is scarce, so I guess I'm only speaking in possibilities and trying to tie things together as best as I can. But this is something I've been ruminating on for a while and I wanted to share it.

I've posted before on these forums what I believe to be the link between χ[chi], Dream Drop Distance and Kingdom Hearts III.

My idea is that Xehanort allowed Sora to unlock the Sleeping Keyholes in order to wake up the Unions, giving him participants for his new Keyblade War. Going deeper, I believe that the collection of Lux is linked to the appearance of the Sleeping Keyholes. Chirithy is a Dream Eater, and Dream Eaters seek out the Sleeping Keyholes.

Xehanort had brought Sora off his intended path as early as The Grid (which can be the second world), which tells me that he let Sora continue diving and unlocking Keyholes on purpose. He could have gotten his vessel much earlier if so desired.

The thing that troubled me about this theory was that there isn't a guarantee that anyone in Daybreak Town is still alive. Surely nearly if not all of them are dead, looking at the Keyblade Graveyard.

So I began thinking about the Realm of Sleep, how it can overlap with the other Realms (The Grid, TWTNW), and about Chirithy, a Dream Eater created by the master of the Foretellers.

I noticed that all Dream Eaters have an emblem. Historically, emblazoned creatures are created, rather than occurring naturally. Emblem Heartless, created from hearts. Nobodies, a byproduct of the creation of Emblem Heartless. Unversed, created by Vanitas. And Dream Eaters, created through synthesis.

Moreover, the interesting thing about Dream Eaters is that they are made of darkness, including the benevolent Spirits.
So what is the Foretellers' master doing creating these dark creatures whose purpose is to seek out the Light (and Keyholes of Sleep)?

My current idea is that the Realm of Sleep is man-made, though I'm not sure by whom.

The running theory right now is that the missing Sixth Apprentice of the master is affiliated with darkness.
The master disappeared mysteriously after bestowing the Books of Prophecy upon the Foretellers.
I can't help but be reminded of Xehanort, who betrayed his master, and may be under the influence of the Sixth. History does repeat itself.

Could the Realm of Sleep be used as a sort of stasis, a sanctuary from the destruction of the Keyblade War?
Was it really the master who created Chirithy?

 

[Grono]

...damn, bro... This trumps my dual wielding Namine theory all the way. But I assumed that the mysterious cloaked figure was, in fact, the sixth member of that group, and that Daybreak Town is, in fact, in the realm of sleep, just like you assumed. So yeah, we're on the same page here That trumps all of my theories for the next week

Although, do you have proof of the nobodies are the byproduct of emblem heartless thing?

 

[The_Echo]

Although, do you have proof of the nobodies are the byproduct of emblem heartless thing?

If a person produces a Heartless, it's come from their heart, making it an Emblem. Purebloods are simply manifest darkness. Sora is the only current example of someone's Heartless being Pureblood (Ansem, SoD is up in the air), though this may have just been an inconsistency, or somehow related to Sora keeping his sense of self as a Heartless.

The only non-emblem Nobodies are the humanoid ones, but they were branded in a different way (the X in their names).

 

[ShardofTruth]

I don't think the Realm of Sleep (or the Sleeping Worlds as it's called in the NA translation) is artificial, I think it's as Ansem, SoD says:
"Dreams hold our memories. Sleep holds our dreams. And darkness - it holds our sleep" (that's probably why Dream Eaters are made of darkness) and of course there is the eponymous "birth by sleep" of BBS which Ansem the Wise refers to by saying "everything is born from sleep", meaning (at least according to Nomura) that everything that vanished from the world can be reborn if you hold on to it. In the essence this realm is responsible of how life and death work in the universe of Kingdom Hearts.

The Sleeping Keyholes are a strange subject on their own since the game doesn't really elaborate on them and Nomura doesn't talk about them. Unlocking them grants the power to awake sleeping hearts and wakes up the world in the process, although it's unsure if they wake up all sleeping worlds or just the one with the keyhole. Since not all worlds were sleeping in KH3D that isn't helping either. Also Yen Sid said there should be seven of them while in the game there are definitely more, some for Sora, some for Riku, but sometimes they don't seem to be Keyholes at all but only gate ways to the Realm of Sleep.
So it's complicated and by linking them to the Unions it's making them even more complicated, so I don't think that's t he case.

I think the master of the Foretellers is sleeping somewhere, creating the Chirithy in the process, something that is possible as shown in KH3D when Sora was sleeping in the real world and Riku was still able to bring his Dream Eaters.

At least it makes me wonder if Ventus is doing something other than just sitting on this chair in Castle Oblivion.

 

[Sephiroth0812]

The Sleeping Keyholes are a strange subject on their own since the game doesn't really elaborate on them and Nomura doesn't talk about them. Unlocking them grants the power to awake sleeping hearts and wakes up the world in the process, although it's unsure if they wake up all sleeping worlds or just the one with the keyhole. Since not all worlds were sleeping in KH3D that isn't helping either. Also Yen Sid said there should be seven of them while in the game there are definitely more, some for Sora, some for Riku, but sometimes they don't seem to be Keyholes at all but only gate ways to the Realm of Sleep.

At least it makes me wonder if Ventus is doing something other than just sitting on this chair in Castle Oblivion.

Since Riku opens such a "keyhole" inside Sora's heart twice during the finale of DDD, I would assume that every living entity (remembering that worlds in the KH universe are also living entities that have hearts) has a sleeping Keyhole.

I'm not sure if I recall it correctly, but I think the thing about the "seven" sleeping keyholes was that this is the amount required to gain the power to awaken sleeping hearts, not the total amount of existing ones.

Ventus' heart should actually have been very busy healing the extensive damage done during the finale of BBS, I don't think that Ventus had any substantial strength left to do something else.
The damage done to his heart was why he needed to seek refuge within Sora's and go to rest after all.

 

[The_Echo]

Unlocking them grants the power to awake sleeping hearts and wakes up the world in the process, although it's unsure if they wake up all sleeping worlds or just the one with the keyhole. Since not all worlds were sleeping in KH3D that isn't helping either.

I'm not sure if I recall it correctly, but I think the thing about the "seven" sleeping keyholes was that this is the amount required to gain the power to awaken sleeping hearts, not the total amount of existing ones.

I'm running under the assumption that there are specifically seven Sleeping Keyholes (for the Realm), as evidenced by the way Yen Sid speaks of them and the fact that Traverse Town has a second Keyhole, and there is one unlocked in The Grid, which isn't sleeping.

When it comes to the Keyholes Riku unlocked, wasn't it said that unlocking the Sleeping Keyholes within Sora's dream gave him the ability to dive into Sora's heart? So he was given a different power from Sora.

 

[Some guy]

I'm not sure about that "only emblem heartless give birth to nobodies" thing. Every heart can become a heartless (with a few obvious exceptions) naturaly or not, and thus give birth to a nobody. Roxas even proves this ...

As for your theory, I don't think the sleeping keyholes have any relevance at all for the story, other than being a goal to attain. And we shouldn't rule out Chi being in a data world yet (which can explain MDG's appearances as well); for all we know, the Chirithys (later becoming all the variants of dream eaters) lived in the real plane of existence before the keyblade war's aftermath ...

I like to think of the RoS as a trash bin of worlds/people, some world were so corrupted by darkness that they couldn't be restored at the end of KH1, thus needing special recovery via the sleeping keyholes. Whether or not all the sleeping worlds were restored at the end of DDD though ...

Also, about The Grid being a "real" world ... Doesn't it make more sense for YX/ Xemnas to be screwing around with Sora's mind? How else can we explain the dream eaters? flowmotion? the dives? the characters not making mention of another boy with a key? the world's story ending where it did (hinting that's how The Grid met it's end)? Also, getting in the RoS was complicated enough but now they can jump through realms for no reason?

 

[Anagram]

I dont agree that the Realm of Sleep is manmade because it is born of sleeping hearts. You can't man-make sleep. However I do believe it's possible to use the realm to revive what has been lost.
From what I've learned in recently, largely from Sephiroths posts, is that one doesn't truly die. Your body may be destroyed, the soul powering it wither but the heart is eternal.
When your gone that heart goes somewhere. Its said they gather in darkness but then again sleep is held within darkness. Birth by Sleep, hearts fall into sleep but can be reborn under the right means.

Of course we know hearts likely go to many places. Hades rules an underworld, heartless are lost hearts and Vexen says in CoM that Maleficent ended up in the dark realm after her destruction. In short the kinda "afterlife" you have varies on world, circumstance, and of course the type of being you are.
For example Xehanort, a human, is terrified of death which means for him and all humans it's likely something not easily overcome even in that universe. However you also have immortals like Hades who quite frankly can't die and then you have Maleficent who has been destroyed yet returned.

But in the end I think the majority of hearts wind up in darkness, aka sleep specifically. Now we have this entire realm born from sleeping hearts. We know "birth by sleep" is more than just a title or metaphor.
Basically put I find it incredibly likely that you can rewake a heart lost to sleep even ones from a bygone war that predates a current universe. All they need is an awakening and a boom their return from they're back in the main reality of the Realm of Light to continue their battle.

I've said it in other threads several times but I find the "Grand Master" (since he/she currently lacks name) incredibly suspect. As much or even more so than their 6th elusive apprentice.
This master knew of a foreboding future yet gave the tools needed to enact it. Tomes from which to summon worlds and power from the future (KH1-current) and the guilds know how to actually craft keyblades. CRAFT them. Not some random slot machine of chance.
I've seen Sephiroth0812 point out several times the keyblade was originally intent for evil so what does that say of the Master who laid the ground work for the foretellers? Does Xehanorts keyblade truly belong to the 6th foreteller or was it actually the lost masters? Did the 6th foreteller take the keyblade or was it entrusted to him? Why did the Master disappear? Was it of his volition or was he disposed in a repeat of Ansem the Wises story?

To many questions, no answers, two mysterious beings with no info. The Master gave each of his foretellers a new name yet only 5 a tome. A new name symbolizes rebirth or a new purpose. What was his intended purpose for the 6th student with no tome?

Sorry about the long post, this master person has been on my mind lately. haha

"only emblem heartless give birth to nobodies" thing. Every heart can become a heartless (with a few obvious exceptions) naturaly or not, and thus give birth to a nobody. Roxas even proves this ...

This^
Roxas is unique but his creation wasn't. (Namines was though) He was a nobody born to a pureblooded albeit weak shadow heartless. The heart forming the heartless was just so happened to be given new physical/human form. (did I get that right?)

 

[The_Echo]

Maybe I jumped the gun on the man-made Realm idea.

Although, if darkness holds our sleep, and sleep holds our dreams, did people sleep before darkness was born?
And I feel like the Realm of Sleep as we see it in DDD may be different from the concept of sleep in general. The way that it connects many worlds and people simultaneously, and can overlap with the waking worlds. I think that's rather suspect.

Also, when it comes to Roxas and Sora's Heartless, I have to claim narrative inconsistency. We know that Purebloods aren't born from hearts. From the darkness within hearts, but not the hearts themselves as Emblems are. This was already established way back in KH1. I figure Sora's Heartless being a Shadow may have been a choice due to it being the most iconic Heartless, and partially due to KH1's nature of having originally meant to be stand-alone.

 

[Anagram]

Although, if darkness holds our sleep, and sleep holds our dreams, did people sleep before darkness was born?
And I feel like the Realm of Sleep as we see it in DDD may be different from the concept of sleep in general. The way that it connects many worlds and people simultaneously, and can overlap with the waking worlds. I think that's rather suspect.

The concept of sleep and the realm obviously differs. The realm is born of hearts lost to sleep, formed form their repeating dreams. Biological sleep is what beings do as they rest. People have dreams while sleeping but they aren't an eternal looping sequence like the realm.
I think "before darkness" might be a misconception. The Xblade is said to be made of both of these elements yet it was a part of the original world. Ergo that means darkness had to exist even then, in some form, to have allowed it's existence.
I believe it was the famous Rebirth theory that pointed out that darkness was likely born in peoples hearts not in general. (aka alluding to the appearance of heartless. heartless are "darkness made real" aka darkness born into a physical presence)

Everything is connected so that's not suspect. However that doesn't mean the Realms role in this isn't. Someone could be exploiting the natural occurrence of the realm itself to spur on their plans.

Also, when it comes to Roxas and Sora's Heartless, I have to claim narrative inconsistency. We know that Purebloods aren't born from hearts. From the darkness within hearts, but not the hearts themselves as Emblems are. This was already established way back in KH1. I figure Sora's Heartless being a Shadow may have been a choice due to it being the most iconic Heartless, and partially due to KH1's nature of having originally meant to be stand-alone.

Your arguing semantics my friend. Purebloods are born from the darkness of ones heart. Emblems replicate that process but behave in the exact same manner. The only difference pointed out in Days was that hearts stolen by the purebloods are lost to form others while the ones taken by emblems are stored.
In the end the factor still remains that there is a heart at the core of this being which is trapped by darkness set loose. The very fact that Sora can recall his experience as a heartless and his return from Kairi hugging that heartless form shows that his heart was within or connected to that shadow.

Edit.
Now that I think of it, didn't KH1s reports first reference the emblem-pure heartless dynamic?
I don't keep track of things but my KH1 memory is still good.

 

[Sephiroth0812]

I'm running under the assumption that there are specifically seven Sleeping Keyholes (for the Realm), as evidenced by the way Yen Sid speaks of them and the fact that Traverse Town has a second Keyhole, and there is one unlocked in The Grid, which isn't sleeping.

When it comes to the Keyholes Riku unlocked, wasn't it said that unlocking the Sleeping Keyholes within Sora's dream gave him the ability to dive into Sora's heart? So he was given a different power from Sora.

First off, the notion that "The Grid" isn't sleeping comes from Young Xehanort and he is not really a source I would count as reliable. Even if "The Grid" itself isn't sleeping, the world in which the computer stands that contains the Grid (which is implied to be the original Tron world from the movies as Radiant Garden's Space Paranoids is a copy program) certainly is.
While I won't put it past the series to let entire realms have Keyholes too, the issue with this is that it was stated that Sora's and Riku's journey only awakened those worlds they went to and not all sleeping worlds as there are confirmed to be others located in the realm by Nomura:

— The setup certainly was different and trickier than before. In their travels this time they were essentially supposed to go to the worlds shut by sleep, but in the end, an entirely different world sets the stage.
Nomura: There are other worlds shut by sleep, from the start which ones they would go to was not decided. Even Yen Sid didn’t know which worlds they would go to. Amidst that, they were guided by two members of Organization XIII to the World That Never Was.

The power Riku and Sora both unlocked is in essence the same: To awaken lost/sleeping hearts from their slumber (if they're healthy, of course). Yen Sid then points out that by gaining this power, Riku now should be able to awaken Sora's heart too. he doesn't mean just Sora's heart alone here.
Here's the part from DDD:

Yen Sid: In your Mark of Mastery exam, you were to unlock seven Sleeping Keyholes.
By doing so, you would awaken those worlds from their prison of slumber, and also acquire the power to free a heart from its sleep.
Riku, you unlocked those Keyholes within Sora's dreams. Therefore, it stands to reason that you now have the power to awaken Sora's heart.

Mickey: You want him to dive back into Sora's sleep? But Master, Sora's heart is down in the darkest abyss.
If Riku's not careful, he might just get trapped down there with him. No... I'll go instead.

(Yen Sid gets up from his chair and walks over)

Yen Sid: And perhaps you may even succeed, Mickey. But there is no denying Riku stands the better chance, having dived into Sora's heart as long as he has.

Note how Yen Sid points out that Sora and Riku awakened exactly those worlds from their "prison" of slumber they opened the Keyholes of and in the process learned/acquired the power to awaken sleeping hearts as well (which in essence is a sort of "revival" spell more or less in the KH universe).
Yen Sid then proceeds to explain that, although Riku opened the Keyholes in Sora's dreams and not in the "actual" sleeping worlds like Sora, it still stands to reason that he received the power nonetheless and thus can awaken Sora's heart.

Mickey then protests that it might be too dangerous for Riku and offers to go himself, implying that Mickey may also have the power to awaken hearts, but Yen Sid, while acknowledging Mickey's capability to do it, say that Riku stands "the better chance" because his connection to Sora's heart is deeper than Mickey's.
The "stands a better chance"-statement also gives us an implication that, while with this power one may be able to awaken (and thus revive) sleeping/lost hearts, it is not a sure-fire thing and it may be possible that the attempt to wake a sleeping heart can fail, in the worst case resulting in the one trying to awake the other heart going down into sleep as well as "he might just get trapped down there with him" suggests.

I'm not sure about that "only emblem heartless give birth to nobodies" thing. Every heart can become a heartless (with a few obvious exceptions) naturaly or not, and thus give birth to a nobody. Roxas even proves this ...

As for your theory, I don't think the sleeping keyholes have any relevance at all for the story, other than being a goal to attain. And we shouldn't rule out Chi being in a data world yet (which can explain MDG's appearances as well); for all we know, the Chirithys (later becoming all the variants of dream eaters) lived in the real plane of existence before the keyblade war's aftermath ...

I like to think of the RoS as a trash bin of worlds/people, some world were so corrupted by darkness that they couldn't be restored at the end of KH1, thus needing special recovery via the sleeping keyholes. Whether or not all the sleeping worlds were restored at the end of DDD though ...

I'm sorry to have to be a little blunt here, but the notion that only Emblem Heartless can produce Nobodies is bogus.
Nobodies can be a rare byproduct of a strong willed heart being lost to Darkness and the only condition that decides if a Nobody is spawned or not is how strong willed the lost Heart was. The type of Heartless resulting from the loss of that Heart is irrelevant.

More bluntly spoken, the seven sleeping Keyholes are/were a plot device to have an excuse for Sora and Riku to visit the Disney worlds in DDD tied together with the "obtainment" of the new power they need to, in Yen Sid's words:

If we are ever to strike down Xehanort, we need the individuals King Mickey spoke of in his letter. We must lead them out of sorrow and slumber, and back to our world. To do so, seven Sleeping Keyholes must be found and unlocked, and a great power retrieved.

This new power to awaken hearts forms a trinity (again alluding to the staple number 3 in the series, alongside 7 and 13) with Ansem's research data and the ominuous "Key to Return Hearts" that is said to be a focal point of KH III (and that games' plot device to visit Disney worlds).
Together, these three "elements" will somehow bring about the process as to how the tormented ones shown in Blank Points (TAV, Naminé, Xion, Roxas and Ansem the Wise) can be rescued by Sora and "return to the world" as both Yen Sid in DDD and Nomura in the BBS Ultimania state it.

"Trash bin" is a little downplaying the role of the RoS I think though. I personally see this realm more like either a sort of pseudo-"afterlife" or a variant of Purgatory.

As I quoted above, Nomura states there are other worlds shut by sleep while Yen Sid said that those worlds who got their Keyholes opened were "awakened from their prison of slumber", implying that there are others.

I dont agree that the Realm of Sleep is manmade because it is born of sleeping hearts. You can't man-make sleep. However I do believe it's possible to use the realm to revive what has been lost.
From what I've learned in recently, largely from Sephiroths posts, is that one doesn't truly die. Your body may be destroyed, the soul powering it wither but the heart is eternal.
When your gone that heart goes somewhere. Its said they gather in darkness but then again sleep is held within darkness. Birth by Sleep, hearts fall into sleep but can be reborn under the right means.

Of course we know hearts likely go to many places. Hades rules an underworld, heartless are lost hearts and Vexen says in CoM that Maleficent ended up in the dark realm after her destruction. In short the kinda "afterlife" you have varies on world, circumstance, and of course the type of being you are.
For example Xehanort, a human, is terrified of death which means for him and all humans it's likely something not easily overcome even in that universe. However you also have immortals like Hades who quite frankly can't die and then you have Maleficent who has been destroyed yet returned.

But in the end I think the majority of hearts wind up in darkness, aka sleep specifically. Now we have this entire realm born from sleeping hearts. We know "birth by sleep" is more than just a title or metaphor.
Basically put I find it incredibly likely that you can rewake a heart lost to sleep even ones from a bygone war that predates a current universe. All they need is an awakening and a boom their return from they're back in the main reality of the Realm of Light to continue their battle.

I've said it in other threads several times but I find the "Grand Master" (since he/she currently lacks name) incredibly suspect. As much or even more so than their 6th elusive apprentice.
This master knew of a foreboding future yet gave the tools needed to enact it. Tomes from which to summon worlds and power from the future (KH1-current) and the guilds know how to actually craft keyblades. CRAFT them. Not some random slot machine of chance.
I've seen Sephiroth0812 point out several times the keyblade was originally intent for evil so what does that say of the Master who laid the ground work for the foretellers? Does Xehanorts keyblade truly belong to the 6th foreteller or was it actually the lost masters? Did the 6th foreteller take the keyblade or was it entrusted to him? Why did the Master disappear? Was it of his volition or was he disposed in a repeat of Ansem the Wises story?

To many questions, no answers, two mysterious beings with no info. The Master gave each of his foretellers a new name yet only 5 a tome. A new name symbolizes rebirth or a new purpose. What was his intended purpose for the 6th student with no tome?

Sorry about the long post, this master person has been on my mind lately. haha


This^
Roxas is unique but his creation wasn't. (Namines was though) He was a nobody born to a pureblooded albeit weak shadow heartless. The heart forming the heartless was just so happened to be given new physical/human form. (did I get that right?)

(sarcasm on)
Well, you can knock someone out and send him/her to sleep...so that would be "man-made" sleep, right?
(sarcasm off)
On a more serious note, Nomura describes that the hearts that Xehanort "infects" are eventually swallowed and captured when Xehanort takes full control and Young Xehanort states to Sora that:

Your heart will sleep forever in the folds of darkness. And your body will be another vessel for me. So light gives way to darkness. Good night, Sora.

So while the RoS itself may not be "man-made" there are certainly "man-made" ways to send hearts there.

It may indeed have been one of my posts where I said that since in the context of the KH universe (and many beliefs in our real world too), death is solely a physical phenomena, meaning loss of the physical body. In the japanese original, the "heart" in the KH series is spelled as "kokoro", which can also be translated as mind, spirit or "essence" in English and describes the immortal part of an existence.
Just like Naminé says in Chain of Memories that memories are "immortal", this may also count for the heart itself as memories are a core part of any heart. So while a heart may be able to be injured, maimed and even ripped apart, given enough time (and other hearts who care), even such a heart might someday be restored.
If you wanted to go for the physical heart, the japanese term would be "shinzou".

It is indeed possible that the Realm of Sleep is exactly the place that allows such restorations/revivals, but since we do not see constant total mass revivals in KH (except during the ending of KH 1), there must be some measures and hurdles in place that prevent a revival "just for the lulz of it" and as far as the efforts Yen Sid and the others go through to make it happen (as well as Ansem the Wise implying that Sora is one of the very few people capable of actually doing it completely), the conditions to even reach the capability of doing so seem to be pretty intricate and high.

Your proposal of Xehanort and his current Seekers aiming to actually revive all those who perished in the first war and force them to fight it anew with the power of "Birth by Sleep" actually made something "click" inside me as I faintly remember Nomura somewhere stating in one of his latest interviews surrounding E3 that x[chi]/Unchained X may eventually reveal/hint at Sora's ultimate role/fate in the whole mess that is going about.
If Sora is really the one who can "completely do it" in terms of revival as Ansem the Wise speculates, Xehanort and co. may actually use this ability to bring back individuals and forces that should better stay gone, namely the ax-crazy Foretellers and possibly even the original wielder (or even the creator) of MX's Goat Keyblade (either the sixth Apprentice or the original Grand Master).
I realize how much this draws similarities to Lord of the Rings (with MX's Keyblade taking the role of the One Ring and its user/creator being an equivalent to Sauron), but the possibility is nonetheless there.

That the Keyblade was originally created for evil purposes was stated by Yen Sid in-game during DDD:

And as for the source of all light--the one true Kingdom Hearts--it was swallowed by the darkness, never to be seen again. As long as it remains there, even the brightest world will have its dark corners.
After all, light begets darkness, and darkness is drawn to light.
For this reason, some decided to use the Keyblade-- a weapon designed to conquer the light--to defend the light instead. These were the first heroes of the Keyblade.

That being said though, the Goat Keyblade could indeed belong not to the sixth Apprentice but actually be the first man-made Keyblade ever as Young Xehanort describes it as "the most ancient one". This insinuates that the thing is even older than the Foreteller's Keyblades.
It's even possible that the Keyblades of the Foretellers were made in direct image of the Goat Keyblade, and to remain with the LotR similarities, served a similar role as the nine rings of the Ringwraiths did to the One Ring.

 

[Anagram]

(sarcasm on)
Well, you can knock someone out and send him/her to sleep...so that would be "man-made" sleep, right?
(sarcasm off)
On a more serious note, Nomura describes that the hearts that Xehanort "infects" are eventually swallowed and captured when Xehanort takes full control and Young Xehanort states to Sora that:

So while the RoS itself may not be "man-made" there are certainly "man-made" ways to send hearts there.

I dont find that exactly the same thing. What you describe sounds more like inducing the natural occurance of sleep. Not something that is literally crafted by human hands.

I think being able to send hearts into sleep is a fairly obvious ability. The method sounds something akin to knocking a person out with blunt force or inducing it with something.
Darkness holds sleep, Xehanort guides them into darkness where sleep awaits. Kinda like he's sedating them when you think about it. haha

It may indeed have been one of my posts where I said that since in the context of the KH universe (and many beliefs in our real world too), death is solely a physical phenomena, meaning loss of the physical body. In the japanese original, the "heart" in the KH series is spelled as "kokoro", which can also be translated as mind, spirit or "essence" in English and describes the immortal part of an existence.
Just like Naminé says in Chain of Memories that memories are "immortal", this may also count for the heart itself as memories are a core part of any heart. So while a heart may be able to be injured, maimed and even ripped apart, given enough time (and other hearts who care), even such a heart might someday be restored.
If you wanted to go for the physical heart, the japanese term would be "shinzou".

It is indeed possible that the Realm of Sleep is exactly the place that allows such restorations/revivals, but since we do not see constant total mass revivals in KH (except during the ending of KH 1), there must be some measures and hurdles in place that prevent a revival "just for the lulz of it" and as far as the efforts Yen Sid and the others go through to make it happen (as well as Ansem the Wise implying that Sora is one of the very few people capable of actually doing it completely), the conditions to even reach the capability of doing so seem to be pretty intricate and high.

I'm sure there is some hurdles. While certain beings like Hades or Maleficent may laugh at death humans, namely Xehanort, does not. This implies to me that even there it's not an easy thing to overcome.

However taking Xehanorts nature into consideration here a scheme to initiate these highly difficult and intricate methods doesn't seem out of his reach, range or character.
He's already cheating time itself without actually altering it.

Your proposal of Xehanort and his current Seekers aiming to actually revive all those who perished in the first war and force them to fight it anew with the power of "Birth by Sleep" actually made something "click" inside me as I faintly remember Nomura somewhere stating in one of his latest interviews surrounding E3 that x[chi]/Unchained X may eventually reveal/hint at Sora's ultimate role/fate in the whole mess that is going about.
If Sora is really the one who can "completely do it" in terms of revival as Ansem the Wise speculates, Xehanort and co. may actually use this ability to bring back individuals and forces that should better stay gone, namely the ax-crazy Foretellers and possibly even the original wielder (or even the creator) of MX's Goat Keyblade (either the sixth Apprentice or the original Grand Master).
I realize how much this draws similarities to Lord of the Rings (with MX's Keyblade taking the role of the One Ring and its user/creator being an equivalent to Sauron), but the possibility is nonetheless there.

That being said though, the Goat Keyblade could indeed belong not to the sixth Apprentice but actually be the first man-made Keyblade ever as Young Xehanort describes it as "the most ancient one". This insinuates that the thing is even older than the Foreteller's Keyblades.
It's even possible that the Keyblades of the Foretellers were made in direct image of the Goat Keyblade, and to remain with the LotR similarities, served a similar role as the nine rings of the Ringwraiths did to the One Ring.

haha Sadly I never quite got into Lord of the Rings but from the context I get the impression that the Goat Keyblade could be the sort master key. The first one to be made in the Xblades image?
That's what I'm thinking as well. This man is a damn brilliant person. Demented as diddly but brilliant. If he put his mind to it I'm sure exploiting the Realm of Sleep wouldn't be impossible for him.

I like the notion the foreteller keyblades are imitations of the Goat ones motif.