Demon Prince
This is the D&D and Pathfinder creature, not to be confused with Warhammer's Daemon Prince
Demon Princes are a specific breed of lesser tier evil God in the Dungeons & Dragons and Pathfinder. As their name suggests, they are Demons who have managed to claw their way to a position of particular power and influence amongst their kind. They may even be worshipped by some evil races as patron gods, though usually they are not "full-tier" gods, thusly allowing adventurers to bust into their homes and beat the shit out of them.
Emphasis on usually: some Demon Princes have managed to secure a position of such power that they are full-fledged gods, so fighting with them isn't a really good idea. The most traditional example in D&D is Lolth, who is not only the Demon Queen of Spiders, but also the patron god of the Drow, a position that saw her promoted to a "corebook deity" in 4th edition. Likewise, in Pathfinder, the Demon Queen of Monsters, Mutation and Miscarriages, Lamashtu, is a full-fledged god and stands as a counterpoint to Asmodeus the Arch-Devil-turned-God.
Some of the more notable Demon Princes include the following:
- Orcus - Demon Prince of the Undead, one of the oldest and most well-known demon princes.
- Baphomet - Demon Prince of Beasts, racial patron god of the Minotaur race.
- Yeenoghu - Demon Prince of Slaughter, racial patron god of the Gnolls.
- Lolth - Demon Queen of Spiders, racial patron god of the Drow.
- Zuggtmoy - Demon Queen of Fungus, and the boss lurking at the heart of the Temple of Elemental Evil.
- Pazuzu - Demon Prince of Air, master of the skies of the Abyss, and a demon with a pretty chill attitude. In D&D, he's the "big shot", but in Pathfinder, he's been dethroned in importance by his ex-girlfriend Lamashtu.
- Jubilex - Demon Prince of Slime, bitter enemy of Zuggtmoy.
- Fraz-Urb'luu - Demon Prince of Deception, a one-time prisoner in the Castle Zygag.
- Graz'zt - Demon Prince of Lust, also known as the Dark Prince, father of Iuz, lover of Iggwilv, sometimes said to be a defected Arch-Devil.
- Demogorgon - Prince of Demons, ruler of the Abyss, lord of all Tanar'ri.
- Lamashtu - A Pathfinder-only Demon Queen and goddess, mother of all "manbeast" races such as minotaurs, harpies, gnolls and medusas.
- Dagon - Demon Prince of the Sea, an antediluvian aquatic horror who serves as Demogorgon's most loyal advisor.
- Kostchtchie - Demon Prince of Frost, racial patron god of frost giants.
- Malcanthet - Demon Queen of Succubi, the latest to hold that title.