I've seen it happen twice now, and though we still easily handle the boss it bothes me that I don't understand why.
Log of it happening from this week:
https://www.warcraftlogs.com/reports...ght=28&death=2
Basically, tank'll be at 100% health; 4+ million, getting overhealed, etc.; totally out of harm's way. Then he'll leave the dream and instantly die to some minor damage hitting him just as he leaves. Here it's Tainted Discharge ticking for "225766 (O: 9859)".
He's entering the dream at full health, so I don't understand why him leaving it sets him to oneshot-levels.
Am I missing something super obvious that everybody else knows about?