The Necromanteion - Act 4 Side Quest

Start Location: Upper City of Lost Souls
Quest Location: The Necromanteion
Objective: Defeat Bonescourge then speak with Podarge

The Necromanteion is a quest that begins in the Upper City of Lost Souls, a town in Act 4, by speaking with Oenobius. For this quest you'll need to find The Necromanteion and defeat Bonescourge, the named quest monster inside, then speak with Podarge to complete the quest.

You'll find the Necromanteion in the Erebus region (pictured below); it's along the northern end of the region on a small platform surrounding by rocks. Bonescourge is found in the large room at the northern most end of The Necromanteion, I would recommend clearing most of this dungeon if you lack good gear as you'll need room to kite Bonescourge around the room.

Once Bonescourge is defeated you'll want to search the alcoves/prison cells for Podarge and speak with him. He's difficult to miss as he is the only shade in the entire area and he has a quest marker above his head. Speaking with him completes the quest.

Bonescourge and Podarge in The Necomanteion.