Anachronos at the Caverns of Time in Tanaris wants you to take Atiesh, Greatstaff of the Guardian to Stratholme and use it on Consecrated Earth. Defeat the entity that is exorcised from the staff and return to him.
This final task will be the most difficult of all, <name>. You have faced and defeated both an Old God and the Lich King's General, but now you must face the hand of Sargeras.
Take the staff to Stratholme. It is there that you will find a piece of consecrated earth: ground where the greatest knights of Lordaeron were murdered. Place the befouled staff upon the holy ground and prepare for an entity of immense power to break from within... Defeat the demonic being that controls the staff and return to me.