Freythorn area boss (The King in the Mists) not starting [Fixed]

Report ID 2596859557

Just reached the area of the King in the Mists, in Cruel. I killed all the mobs but the boss doesn't start, the black mist animation is there but the actual fight doesn't trigger.

Leaving the area and returning doesn't fix it, I have to create a new instance.

Edit: Nevermind I missed an altar...