Hades

Hades, the Gray Waste, is the outer plane of pure neutral evil, at the bottom of the Wheel. Its three layers are colourless and joyless, and the longer a visitor stays, the more their memories, feelings and will drain away, until they become part of the grey. Yugoloths and night hags prowl it, and it is a battlefield in the war between devils and demons.

Reaching it. By a rust colour pool in The Astral Sea, a dark red curtain in the Deep Ethereal or a plane shift fork of iron tuned to E.