So it seems that this happens during one of his phases and it can happen randomly at any time. I’ve had it happen when he’s at 10% and when he’s at 90%. So obviously, the quicker you can kill him the better in order to avoid him randomly hitting the phase. The only advice I can offer is to not kill the adds if he phases out and let them kill you, then you can try again without re-clearing the entire dungeon (this has limited hardcore viabiltiy).
1 Like