Originally Posted by
Narasinha
Mastermind, with the help of Emma telepatic device altered Phoenix perception of reality over several weeks/months. They made her think she was living another life to the point that she wasn't making the difference between the reality and the illustionary world they imposed on her. She wasn't even recognizing her friends.
While under their influences, she behaved in a totaly different way than her normal personality. She even believed she was someone else. It also was shown, several time, that Phoenix real personality was still there and fighting against the Dark Phoenix persona.
By all account, it's the signs of extreme schizophrenia (or possession).