Dispater, &, is one of the demon princes. He is lawful and has no lair. A spellcasting monster may summon him in Gehennom with the summon nasties spell, or a lawful demon (usually a prince; Asmodeus, Baalzebub, or Geryon) can gate him in. He is generated with a wand of striking.
He will not generally use his spellcasting attacks before he uses up his relatively harmless wand of striking. Therefore it is a very good idea to kill him as quickly as possible.
NetHack's incarnation of Dispater is based on the unique archdevil of the same name found in Dungeons & Dragons source books. The "City of Dis" mentioned in the Encyclopedia entry does not appear in NetHack itself, although some variants add a lair inspired by it.
In the source material as well as in NetHack, Dispater's apperance is that of a handsome human about 2 meters (ca. 7 feet) tall. Unlike his NetHack incarnation, the D&D Dispater also has two small horns, a tail and a cloven left hoof.
In the real world, Dīs Pater was a Roman god of the underworld, later subsumed by Pluto or Hades (Hades was Greek). Originally a chthonic god of riches, fertile agricultural land, and underground mineral wealth, he was later commonly equated with the Roman deities Pluto and Orcus, becoming an underworld deity.
Dispater is an arch-devil who rules the city of Dis. He is
a powerful mage.