His is a new Raid World Quest.
After you will kill
go left and go down on Sunken Stair, Behind Statue you will find him, Is before
Just in case i done video if some one will need
Tomb of Sargeras: The Dread Stalker Raid World Quest
Get to the room the marker is in, and then look up from wherever you are on the spiral staircase. It is hanging from a dish in the hand of a statue.
If it weren't a blood-sucking demon, it would be kinda cute.
Armies of Legionfall Reputation Guide
Legion World Quests Overview