Magister Idonis wants you to venture into Deatholme to slay Masophet the Black, Jurion the Deceiver, Borgoth the Bloodletter and Mirdoran the Fallen. Report back to him in the Sanctum of the Sun in the Ghostlands after you've completed this task.
Masophet the Black, Jurion the Deceiver, Borgoth the Bloodletter, Mirdoran the Fallen. Learn these names well for in the Ghostlands these Scourge leaders are second only to Dar'Khan himself. Slay them and their mindless armies will be thrown into disarray.