Carceri

Carceri, also called Tarterus or the Red Prison, is the outer plane between neutral evil and chaotic evil. It is a jail for traitors and the treacherous: six layers, each a string of planetoids glowing dull red, every orb smaller and further from its neighbours than the layer above. Escape is said to be almost impossible.

Reaching it. By a olive or red colour pool in The Astral Sea, a gray-green curtain in the Deep Ethereal or a plane shift fork of iron tuned to E♭.