&

& represents any major demon in the game. They are quite strong opponents, and can summon other demons, with the exceptions of incubi and succubi. Sacrificing your own race can summon a demon lord, as can angering your god when on an altar (by praying with negative alignment, I think), if you are chaotic. The three riders are also represented by the & symbol. Monsters that can be represented by &:
 * bone devils
 * erinys
 * hezrou
 * horned devils
 * ice devils
 * incubi
 * mariliths
 * pit fiends
 * succubi
 * vrocks
 * water demons

Unique Monsters

 * Asmodeus
 * Baalzebub
 * Death
 * Demogorgon
 * Dispater
 * Famine
 * Jubilex
 * Orcus
 * Pestilence
 * Yeenoghu