Hades. Annwn. Niflheim. The land of the dead. This colourless place is the Outer Plane of Neutral Evil, home of the yugoloths or daemons. All colour fades here, leaving all things grey. The gods of the dead live here, hoarding souls until the end of time. This is the underside of the universe, the last resort of all. The layers are:

Oinos, the wasting plain. Blood War battlefield and home to the daemons' capital, Khin-Oin, a tower made from a forty-mile spinal cord ripped from a dead god. Disease and decay rule all here.
Niflheim, the Norse underworld. Here dwells Hel, the death goddess, and Arawn, the Celtic king of the dead.
Pluton, the Greek underworld. This is where Hades holds court over the pale shades, guarded by Cerberus, the hound of Hades.