You must defeat the lords of Scourgeholme if this place is to be free of the Scourge's grasp.
Salranax the Flesh Render lords over the temple on the first tier of Scourgeholme. Nearby Salranax you'll find High Priest Yath'amon preaching at the spirit tower on the second tier. Underking Talonox holds court at the Pit of Fiends, up the high road.
These are formidable opponents and must be approached with caution. Use this war horn should you require aid. The death knights of Acherus will come.