The Burning Legion has proven its capacity to corrupt mortal souls time and again.  If I'm to entrust you with the knowledge you seek, I must have proof of your allegiance.$B$BA demoness known as Sathal has eluded me for a long time as she's gifted with an incorporeal form.$B$BShe watches over the Wrath Priestesses as they do her bidding at Forge Base: Gehenna.  Slay one of her minions and spill her blood on the ground.  This affront should cause Sathal to materialize temporarily.  Slay her while you can.