I just defeated myself, Semaj, Angelo and the rest and seem to be trapped in the room. Am I missing something? or is it possibly a bug

It must be a bug, try reloading.

probally a bug. try again. there is also tamoko and some wizard (perhaps that is semaj). you sure some didn't turn inv?

did you backstab the wizard?
if you backstab a mage to death and his contingency casts Mislead this can result in a completely invisible, invulnerable copy of the mage.
try shooting some fireballs or cloudkills around to find out where the enemy is, or try detect evil

Try casting a Planetar, they can see invisible targets.

True Sight works well too.