Travel to the Altar of Damnation, and use the Unfinished Headpiece while standing near Gul'dan. When the headpiece is energized, return to Parshah in the Magma Fields.
The staff is nearly finished!
All that remains is to imbue the headpiece with power flowing from the echoes of Gul'dan in this accursed place. To the east, near the Fel Pits and south of the Deathforge, is the Altar of Damnation.
There, fragments of Gul'dan's past are trapped. When you carry the unfinished headpiece of the staff into the area around the altar, don't be surprised if some of the entities react to it. You will be carrying a dangerous object with strong connections to the cataclysm.