Wow.. so a Vanitas-like form of Riku?
That'd be a bitch to kill, knowing SE.
More like Sora's darkness gaining an own consciousness like Vanitas did.
It would be Sora's darkness turned into a living being with a heart of pure darkness by cutting Sora's heart just like Ven's.
That thing
would gain Riku's features if Riku would be goaded into performing the same saving grace newborn Sora's heart did for Ven's in the BBS Prologue.
It's all only pure speculation though, never shall forget that.
i hav heard that it is both vanitas or anti-sora like dream eater. i really hope sora doesnt become a dream eater. vanitas makes perfect sense. it is like his original form before sora and ven were connected. so maybe this shows that the ven/sora bond was broken and he has returned to his original form? im just theorizing here... but wasnt the part of ven that is in soras heart the same part that got put into roxas. so the only thing i can think of is that sora was turned into a heartless, recreating roxas, seperating ven from sora, and making vanitas no longer have anything to do with sora (well, less to do with sora). then vanitas could return to his original form.
These are also only speculations, and I doubt the connection between Ven and Sora can be broken that easily, judging by how tightly knit it is.
Don't make the mistake to say Sora/Ven-Heart connection = Ven's heart in Sora's.
The connection between their hearts was already there way before.
Sora cannot have been turned into a heartless though as he sits in that Org throne. If he would be turned into a heartless he would become a shadow again.
EDIT:
For all we know that could be the true embodiment of all of Sora's anguish with the ability to manipulate its form, size, and or time. (Hoping it has ties to the MF from BBS)
That is my very first theory on that "thing". That it is an exceptionally strong, humanoid nightmare that is all the pain and torment of Terra, Aqua, Ven, Xion, Naminé and Roxas combined.
Further I theorized then that the black Ven-armor with the nightmare-symbol would be an embodiment of Ven's torment alone.
The MF was already confirmed though, it's the golden eyed teenager running around who looks like a young Master Xehanort.