The Pillar towered over a mile (over 1.6 km) into the sky of Avernus. It comprised a wide variety of skulls, ranging from the tiny to the size of a house. The majority, however, were of demons, the enemy of the devils.
It stood by the ledge around the layer of Avernus, near a low point where access to the next layer down, Dis, was possible via an exceptionally tall spire in that city. Near the base of the Pillar of Skulls was a huge cave mouth, in which laired the dragon-goddess Tiamat, who guarded passage between the layers here.