So after beating Taramiel and seeing the PC stuck with Irisviel I am unable to progress, the game tells me to find the Curse book in the underdark but I can't find it anywhere additionally there appears to be a new mob that i can't interact with upon leaving the dungeon area, i also can't return to the dungeon area after i leave. Is this a bug?

There should be a Succubus sprite after you beat Taramiel and you have to defeat it so she can dropped the book.