After this last patch this quest is totally fubared. After the end boss gives the typical speech, it teleports to middle of the area, but before it even lands another copy of it spawns right on the same spot. So I end up with two bosses now and yes, both will aggro and attack when I engage.
One of them can be brought down to 0 HP but it will not die, instead it will just keep jumping around and nuking me, while other nukes me, teleports around and to the pillars and summons waves and waves of undead. When I kill the undead then one of the bosses imply disappears, leaving other one there. Guess which one? Yes, the blasted invulnerable one! So in the end I cannot complete this quest because it's bugged to oblivion. And since I cannot complete it I cannot advance any further either, which means no Coldharbor, no nothing.
Great work there, really Zenimax.. Now can we get some bloody customer support reps IN GAME so that they can pop over and remove the extra sucker?