Stretching all the way across the Material Plane, Zylogmus Eternus is the "infinite labyrinth." Housing all manner of horrors, and plagued by a terrible curse, Zylogmus Eternus is one of the worst places you can possibly find yourself.
Zylogmus Eternus is an enormous maze of corridors and tunnels spanning thousands of miles, built on the lower surface of the Material Plane. It has undergone countless modifications and iterations over the eons, and is a hodgepodge of new and old. Factions control various parts of this megadungeon, vying for power, but their borders are constantly shifting, as the fight for territory is fought nearly ceaselessly.
Dividing all of it is an invisible grid. Nobody knows exactly where the grid lines pass through, but once every month, a random person from within that grid doesn't wake up that night, dying painlessly in their sleep. If someone is sacrificed (burned alive) in one of the eternally burning braziers scattered through the labyrinth, that grid square can avoid having a random person killed.
Up above, there is the Meat Grinder — an impenetrable ceiling of twisting gears and blades. No one has ever gotten through this alive to live to tell the tale. Below lies the abyss — a less explored, more dangerous dungeon. All sorts of rumors exist about what lives deeper, but as with the Meat Grinder, few make it back.
Scattered throughout the dungeon are the sacrificial braziers, used to burn those who are given over to the dungeon.
The shadows of the dead come out at night and kill those who are still wandering the hallways. Curfews are enforced.
Many governments have established sacrificial pools — they sacrifice those who are weak in order to ensure the safety of themselves.