(Mostly just mad rambling about stuff, getting it down)
The Demon princes; Born from the Corrupted half of Ilifos, they rule their own plane with Demons. The Abyss is like lvl 2 hell, and the Second are like the Obyrith. However the Demon princes are waaaaaaaay more active than the second. Apollyon is actually on Ageon, and Dagon is a servent of the Apocalypse cult of the Leviathan. Abaddon is protector of one of the few things you would go into Hell for; The Fountain. It grants immortality (no strings). Azuvidexus is the primarch of physical destruction, worshipped by both the Dothra and the *Sahuagin* Alocer is the Graz'zt of Ageon. Baphomet is more fridge/realistic horror than the rest, being very easily summoned and being the current Prince of Demons. (After disposing of the former one, whom bore an uncannily close resemblence to a certain angel). Belphegor is the Queen of Hell, and refuses to be summoned whatsoever, limited cult. Erebus is the most 'Friendly' of Demons princes, as he doesn't really take your soul. He's very reasonable as far as they go, and is more closely LN than LE. Ereshkigal is the consort of Abaddon, and Prince of falsehood/things you do in the dark. Izyagna is Queen of Witches, and patron of Dark Saints.
Leviathan is the former prince of the apocalypse, he seeks to regain his power after being ruined by the current Prince; The Demogorgon. (Not 'Demogorgon', THE Demogorgon. Not quite a Demon prince, not quite a Sephigant. Created when one of the Second was raised from the dead by Ageon's weird style magic (Which has both Positive and Negative energies powering it, making Necromancy a whooooole bundle of strings) It was alive as a Second could get, so it attempted to control Hell. Suffice to say, it almost managed it, but failed when it took a consort, and due to the nature of the Abyss (Ho Ho HOooo)
The Second are mostly incompatible with Demons, except in three total cases. The Demogorgon was born instantly, and slaughtered the Second, and the consort. It then challenged Leviathan for it's title, and pretty much won handily.  It is worshipped as a patron of deicide, due to it also, after gaining the title of ruination, forcing it's way out of hell onto the material plane, and shitstomping a small pantheon of mortal gods.) Lilith is the Queen of the Apocalypse, Mother of monstrosities.  While I'm thinking about it, Hell is D&D's Abyss, The Abyss is Ghenna/Carci, and A third as of yet unnamed plane is The Nine hells of D&D.
Demon Princes DO grant spells/power, but they actually grant POWER, not magical usage. The price isn't actually a mortal's soul, due to that being the domain of Variun, and the Demon Princes know better than to mess with her (That's the 2nd case of Second/Demon) Back on track.
Marbas is the Demon prince of Knowledge/Archives, and has one of the only three Books of Mors efformarunt.  Marbas is one of the nicer Demon princes, taking payment in gold and blood to view it's impressive collection, but woe betide you if you damage any of it's books. Resembles a monsterous Insect-thing.
Finally Yeter'el is the Demon prince of mortality. For him, mortality is real, being the only Demon prince that can age, Yeter'el is actually the fifth of his name, having died by age four times hence. It's easily the most bitter Demon prince, and hates immortals with a passion.