Starting from the few informations we have... (?)... I’d like the story to be about a conflict between Bhaal and one of his children, an illithid returning from wild space and not surrendering his divine essence to the god of murder. The player(s) character(s) and NPCs could then just be random heroes involved, siding with one faction or the other.
I’m not a Larian dude, you wish I’d be!
Or... the main player character could be Lilarcor, in a quest to redeem itself from bloodlust and frenzy and... (fails)... just kill, kill, kill!