So I've gotten pretty far in my latest RNS game featuring an F/T and Aerie, far enough to attempt to beat up Firkraag. His dungeon wasn't any problems, but then when it came to the big boss himself, I wanted to try that Magic Resistance -> Lower Resistance combo that is supposed to work so well.
Aerie: Magic Resistance Firkraag
Then his bloody circle disappears!!! 
Making it impossible to target him with anything whatsoever. He's there alright, happily killing my summons, but I can't target him with melee attacks, can't target him with spells, can't do nothing. I've seen this phenomenon before actually, sometimes happens with some of the eyeless priests you meet in the Beholder lair, but that was triggered randomly by any sort of spell, and they also got their circles back after some time, this didn't happen with Firkraag.
So ehr, I'm usually pretty hard on myself, but there really was no way I could go on with that fight, and I just couldn't bring myself to start all over from Irenicus dungeon because of a bug like this.