Use the Divination Scryer in the heart of the Great Ossuary's basement in the Scholomance. Doing so will bring forth spirits you must fight. Defeating these spirits will summon forth Death Knight Darkreaver; defeat him.
Bring Darkreaver's Head to Sagorne Creststrider in the Valley of Wisdom, Orgrimmar.
Death Knight Darkreaver lords over the Great Ossuary in the Scholomance. He threatens to twist the spirit realm to do his own bidding, and he must be stopped! This is where the scryer you helped make comes into play.
Use the divination scryer in the heart of the Great Ossuary's basement to call forth Darkreaver's spirit minions. Defeat the spirits and the death knight himself will appear. <name>, bring me his head! Only his demise will reverse the damage done to the spirit realm!