Zul’drak quest : Disclosure : Malmortis still buggy as of 08/Jan/2020
upon teleporting to the coffin and opening to get Malmortis to spawn, he will then take damage from an invisible npc until his death.
There is a workaround. Spawn in Malmortis and jump down below, stand by an elite until you see it being attacked, then head back and respawn Malmortis, follow to completion.