Challenge rooms that occasionally get embedded into Gehennom levels. They are sealed off somehow, perhaps with solid stone you need to dig through to enter. They are not intended to be required, and contain some sort of reward. Rooms might include:

• Demon den: one or more demons guarding some gold and gems. (In a cavernous Gehennom, we can expect that most of the mineral wealth has already been mined out by millennia of demon activity.)
• Seminary of Moloch: contains an altar to Moloch, one peaceful priest of Moloch, and multiple hostile priests of Moloch.
• Fake wizard towers: the standard 7x7 footprint consisting of moat and tower, with an amulet in the center of the tower (possibly EPI: this could be a fake Amulet of Yendor), and a few monsters inside the tower. The flavor basis for these is that they are the Wizard’s earlier settlements.
• Hostage room: contains a lot of monsters guarding a prisoner or player monster hostage. The reward here might be that the monster auto-tames when freed.