House of Gruumsh
Gruumsh lives in a gigantic obsidian fortification dubbed the House of Gruumsh by his followers. Defended by the spirits of the greatest orcen warriors who ever lived, it is accessible only by an extra-planar set of obsidian stairs. The castle is within the nine hells, but where the stairs begin is a mystery.
Pictured: House of Gruumsh set against a large mortal castle for scale.