The Realm of Pain is one of the seven realms of Hell, and the domain of Duriel. Out of all the realms, this one is one of the most chronicled in ancient artworks. Deckard Cain suspected that this was because the realm most closely resembles the torture chambers of the mortal realm and therefore is most easily understood and depicted by human artists and writers.
Whatever the cause, the realm is described as a cavernous area fitted with increasingly sadistic and grotesque torture devices, from towering machines that house engines of brutality to tiny mechanisms designed to inflict agony beyond imagining. Thousands of captive demons were kept in these machines, giving Duriel gratification from the pain inflicted. It is said that the other denizens of Hell, including Andariel, avoid this realm, though they delight in the music of the screams that ring out from it.
The realm is located adjacent to the realm of Andariel herself, and the two overlap to a degree. It has been suggested that those driven mad by Andariel's realm, driven to seek physical torment, willingly give themselves over to Duriel's domain.