Happened to me once, needless to say we were doing poorly but were somehow still alive. When he goes to the platform to place the relic for the runner initially, when he is enraged he immediately goes to the stagger phase. We didn't know he was going to do that and we didn't stagger him so we wiped. Idk what happens after that.

Ive never seen him enrage since I will either beat him or the entire team will wipe before it happens