Arsenic Vault

The Arsenic Vault is a vast mountain-turned-fortress that can be found in Malebolge, the sixth layer of Hell. The Arsenic Vault is connected to the Material Plane by the Magrevore, a portal hidden in the Darklands somewhere beneath the town of Khoka in the desert nation of Qadira. The Magrevore is guarded by Mardehzuk, a powerful pit fiend worshipper of Moloch who uses a network of hryngar to secure slaves that he then sacrifices to the Magrevore, channelling them to Hell via the Arsenic Vault. The Arsenic Vault itself is an enormous mountain of solid metal that, over uncountable eons, has been slowly carved by Malebolge's devils into a single, massive fortress.