I could seriously help you there. Just a brief WIP i got from that concept alone.
As far as a base for things. The keyblades originate from the very force that brought them in, Light/Karu, Twilight/Atsu, and Dark/Kura. The gods created the three realms (by like omniverse terms its known as "planes" in a different format)
Kingdom Hearts was an atonement or means of birth, something greater than even them. Even these "gods" possessed hearts. From their hearts in particular did the keyblades manifest. The Light, The Inbetween, and The Inverse. One by itself could easily open or close kingdom hearts. So depending on who was greedy, that theory of the keyblade bringing ruin to the world stands true.
Kura would be the instance of what happened to Riku (in KH Final Mix). He allow his heart to be swallowed by darkness however he retained his sanity, emotions, and body. And was able to manifest the first heartless, this was possible by opening kingdom hearts to release the darkness to invade into the other realms. (Keyblade war number 1) Take into consideration that these realms were uninhabited since this is the pre-work of history in the KH universe. So it was virtually Karu and Atsu against all odds.
On their caliber, they practically obliterated the heartless and stood to confront Kura. Ultimately, he was then sealed behind kingdom hearts to this day which is known as "True Darkness" to the point where its fear embraced. (Remember when mickey found the key behind the door to darkness? Hm? HM? lol) It's later assumed through the stories that they regressed into the keyblades themselves. And he whom wielded them passed on the legacy of the gods are were deemed godlike. So your prophet idea kicks off from the genuine 3 continuing the story of the three.