Altruis the Sufferer wants you to obtain Freshly Drawn Blood from a Wrath Priestess at Forge Base: Gehenna in Netherstorm. Spill it on the ground and slay the Avatar of Sathal. Return to Altruis when you've completed this task.
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.
A demoness known as Sathal has eluded me for a long time as she's gifted with an incorporeal form.
She 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.