The Archdevils of the Nine Hells! Most of the D&D background stuff I’ve read I can take or leave, but these guys have really stuck with me.
From the top:
Bel, Archdevil of Avernus. Military man, latest in a long line of Pit Fiends promoted to serve as Hell’s battle commander. Good with strategy, hopeless at politics. Ambitious, with a bad temper.
Dispater, Archdevil of Dis. Real paranoid, seldom leaves his tower. Obsessively builds Iron golems and spinning blade traps. Never forgets an insult.
Mammon, Archdevil of Minauros. Lives underground on a big pile of wealth. Looks more like a big snake with every passing year. Generally regarded as insane.
Belial and Fierna, Archdevils of Phlegethos. Married couple. Fight all the time and cheat on each other with strings of disposable partners, otherwise oddly devoted to one another. Their lavish parties are worth a visit, as long as you remember the complex etiquette.
Levistus, Archdevil of Stygia. Handsome rogue, always up for a good time. Passes for human, rumoured to have started that way. Currently entombed inside a giant iceberg, but still conscious, and very angry.
Glasya, newly installed Archdevil of Malboge. Rumoured to be Asmodeus’ daughter by an unknown mother. Highly efficient and devoted to her job. Invites gifted, evil people from all planes to serve as her lieutenants. Only behind closed doors does she reveal her true taste for terrible cruelty.
Baalzebuul, Archdevil of Maladomini. Once a great Angel of the High Heavens, his fall from divine grace was followed by an even greater fall after Asmodeus cursed him with the form of a giant, hideous slug. Now he weeps with self-pity in a palace made of his own slime.
Mephistopheles, Archdevil of Cania. Even other Devils start to feel uncomfortable after too long in a room with him. Doesn’t talk much, but everyone thinks of him as a marginal ally. Collects interesting scientists, artists and wizards from all known worlds, and puts them to work on mysterious projects, hidden from view.
Asmodeus, Archdevil of Nessus, and ruler of the Nine Hells. When people talk about ‘The Devil’, they mean this guy. As powerful as a God, with the attitude to match. Though he is still bleeding and bruised after being cast down from Heaven in ages past, his voice is like a thread of warm honey in the ear, filling the listener with assurance- and absolute obedience.