Dispater, &, is one of the demon princes. He is lawful and has no lair. A spellcasting monster may summon him in the 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.