@antijingoist Oh yeah, the plot there is that he was one of Mystra's Chosen as a kid and then later her lover for awhile. Then she discarded him.
He tried to get her favor back by messing with some magic forbidden by her - which, yeah screwed him up, but also angered her. Then, later in BG3 she tells him he can make it up to her by blowing himself up at the Big Bad.