Also reported on Steam: 

Me and my friends got to the Eriaudy's fight (for the first time), we blasted through the first phase of the fight, even tho it was missing hp bar gui, but, on the second phase, it just started draining our hp regardless of the place we were at, no, it didn't hit us with melee, nor the falling stones, it just sapped the life out of us.

Only thing I can think of is he sends a twirling mass of darkness that seeks you out and sticks to you, you need to outrun it for it to stop and I think if you didn't see anything then perhaps that just didn't render properly?

Well, i didn't see anything around me, i was playing as a guest, host didn't see it either we just randomly collapsed