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Your parents didn't love you enough, did they?
Well I'm particularly fond of the "Dis-soul concept.
Dis basically means to go in a different direction and could mean separated souls.
I think they may be called something else but as it states in the Ansem reports there are three elements of life. Heart body and soul. We've seen the no hearted heartless, the no body having nobodies and all thats left is the no souls.
It would fit with Xahenorts studies could involve the soulless.
Perhaps he found a way to remove the souls from people. The soldier seen in the secret ending could be his own soul removed and perhaps the result of this are the orange colored eyes.
Terra may have known about it or even dabbled in it which would explain the change in eye color