You know later on you will defeat Malistare, this is just him coming from the dead. He is fought with four other people as a final boss in the game. He is either a rank 12,13 or 14. I have watched someone battle him and he is VERY powerful. But by then you are around the time where you have all the cool and dangerous spells
Malistaire the undying is Malistaire himself but he has been revived by Morganthe (new main villain) and turned into a wraith. In an attempt to spoil to much for you all you really need to know that he truely is stronger than ever and now it's round two.