Minauros
The Third Layer of Hell is Minauros. The layer of Minauros teeters on the edge of ruin. The realm is a great swamp, interspersed with cities and fortresses that are in constant need of repair, upkeep, and replacement. Time and again, structures built on this layer are left untended and are eventually drawn into the bottomless muck of the swamp.
Mammon refuses to spend any more coin than necessary to keep the soul marketplace in the Sinking City functioning. Devils and fiends from across the planes gather here to trade souls. The place bustles with activity as caravans arrive and depart and merchants haggle over their wares. Buildings rest haphazardly atop the ruins of those that have sloughed into the muck. The roads are little more than huge stones sunk into the swamp, needing constant replacement as they slowly submerge until the mud consumes them.