This error was encountered on PC-NA in Veteran Maw of Lorkhaj on Wednesday evening.
During the room duality mechanic, where team members must stand on the appropriate side of the floor or die, Skinrai was killed. Vashai remained with 3-5% Health. The duality mechanic did not play, as Skinrai died before its animation completed.
Immediately after his partner's death, Vashai began his map-clearing execute. All those in range (6 players) died immediately to the mistimed attack.
Normal behavior is for there to be a delay after his partners death, the remaining boss to teleport to the center of the room, and only then begin his execute mechanic.