Tried to complete the encounter via the normal route several times tonight, but the Wandering Plague of Siphoning was hitting the entire group while he was casting his beam (as opposed to afterwards) with no icon to cure.
That happens occasionally. It's hard to tell, but it *feels* like it's related to dots or procs triggering as he goes immune and teleports. Something triggers and hits him just as he's porting, and the encounter bugs out. Any time he's been cranky, we've let up, and just DD'd him across every 25% line on the next pull, and he's behaved.
