Find the wards in the Sinking Sands. The wards are shaped like large dark Ys and spawn randomly. See the talk page for random locations found by other people:
Examine the pyre in the Sinking Sands at ( -603, -130, -1143 ) Copy/waypoint -603, -130, -1143. Warning: You are now on a 3 minute timer to complete next two steps.
Kill 5 level 50+ skeletons. There a four immediately around the pyre and more packs slightly to the north.
Return to the pyre and examine it to spawn the Ghost of Hadal before the timer ends.