Balor- Generally the more powerful (or nonsensical) a monster is, the more inclined I am to want it to be a unique creature rather than a race or type. The Balor is no exception to this rule.
I don’t really see Balor as a demon lord so much as someone’s long-suffering strong man (just not Orcus, they’re both too big and red, it would be overkill). Maybe Graz’zt sends him out on jobs when he doesn’t want to get his fancy, six-fingered hands dirty.
Hezrou- I can’t actually picture one not sounding like Beastman from He-Man.
Marilith- Awesome enough that I don’t feel the need for them to be unique creatures, but there also shouldn’t be too many. A small group of sister mariliths, like the gorgons in Greek mythology, would be great. They could exist outside the hierarchy of the Abyss in some out of the way place where even powerful demons fear to tread. They should know many powerful secrets and both demon lords and adventurers should seek their council from time to time. I also like the idea of their being some kind of special condition or cost for their help, like one member of the group that seeks them has to be left behind as a tribute.
Quasit- Quasits are basically the D&D equivalent of that thing that sat on Jabba the Hut’s shoulder and cackled. They just don’t have the same charm as Imps.
Retriever- An entirely likeable monster. Trespass in the ancient shrine of some demon lord and these guys will come in the night and start snatching people from the camp and dragging them into the darkness to die.
Succubus- A succubus doesn’t have to settle for being just a monster, she can be an entire adventure. Since they’re among the more human of the non-lord demons, it’s easy to see a succubus as a recurring NPC, maybe even one of those foils that helps sometimes and sabotages other times.
The Big Names:
Baphomet- At first I wasn’t all that impressed, but then I saw it: HE IS IMMUNE TO MAZES. I don’t think it needed to be said, but I’m glad they said it. I’d like to think that the decision rose from the fact that every time adventurers came, they were sticking him in mazes just to be clever.
Dagon- Nothing wrong with the idea, but I would change the name. Unless you want to throw in Cthulhu and mother Hydra or listen to your NPCs make endless Lovecraft references.
Demogorgon- I want to like him. I really do. I think it’s the pictures that I can’t get behind. The two heads aren’t so bad, but neither the lizardy heads in 3.0 Book of Vile Darkness nor the Baboon heads in Fiendish Codex I really give off the whole ‘Baddest Demon Ever’ vibe. Maybe if he were a two headed dragon, or a guy with one head and two faces it would be easier to respect him.
Fraz-Urb’Luu- Do you ever get the feeling the people who write these books are just dicking with you when it comes to names? Deception seems like a really strange choice of gimmick for a stocky, bat looking guy, but hid Deception ability is fantastic. Even better than immunity to mazes. Basically, he uses it to convince other demon lords that they’re been summoned, then runs away. He should be a campaign long recurring villain, like some sort of zany demon Skeletor.
Graz’zt- He’s a classic and I can’t argue with that. Sure, dark man in the woods who consorts with witches is an old gimmick, but it’s a good one.
Juiblex- Today is the first time I realized that his name isn’t spelled Jubilex, so I’m still a little stunned. I love this guy though. Sure he’s a little silly, but he definitely has real Lovecraftian terror potential in a not-as-obvious-as-Dagon way.
Kostchichie- I don’t like that he’s a frost giant because his name sounds way too close to Koschei, who is clearly the basis for the lich.
Malcanthet- Ignore the surprisingly tasteful and not overwhelmingly sexy picture. The queen of the succubi should look like Elvira and act like Heidi Fleice. I’d like to see her used as some kind of extraplanar Circe that lures men in with her feminine wiles, toys with them until she gets bored, then turns them into beasts or monsters that reflect their soul.
Obox-Ob- Primitive horrible, filled with rage at the loss of his ancient power and willing to do anything to get it back. That is the stuff of awesome NPCs. The way I see it, there are two really good ways to go about using him.
First, he can be the mastermind behind everything that’s happening. Eventually, after journeying to the Abyss and battling demon lords, the party discovered that Obox-Ob has been the one pulling the strings all along and that he is their real enemy.
The other option I like is having him as the sort of enemy-of-my-enemy. In this case, the party will work with him knowingly or unknowingly and either directly or indirectly. The paladin’s best friend or the party guide could be the demon lord in disguise. Hell, demon lords have time on their side and he’s ancient already, why not make it even more challenging for the party and have him be someone one or more members have been close to since childhood. While they always knew him as a dear friend of mentor, he was just sticking close to them so that he could raise a champion that would eventually help him regain his throne.
Orcus- The fact that he’s fat is never illustrated well enough. While most monsters lose their animal fierceness by packing on the pounds, the idea of the lord of the undead as a lazy, obese creature that constantly feasts on the sins of the dead is much scarier than someone who looks like they’re just going to straight up punch your head off. Rather than being completely animal or alien, his fatness makes him a mirror for human vices and that carries a lot of power.
Pale Night- One of my favorite of the demon lords presented in the Fiendish Codex. If I were running a game featuring her, I would up her CR off the charts and make her into a sort of primal goddess among demons, like Nyx or Chaos.
Pazuzu- Demon lord of birds and shit. What makes him so special? He has wings. So does everyone else. What else? He has a beak. Are you kidding me? I really don’t think it’s necessary to have demon lord for everything, but I do like that if you say his name three times, you create a temporary bond to him. Candyman is one of my favorite horror films, so anything that reminds me of it does earn a few points. Oh, and you know, Hastur Hastur Hastur.
Yeenoghu- For the more primal humanoid races, I really like the idea of a demon lord or powerful unique monster serving as their god. A lot of gods really seem like they’re phoning it in with the divinity thing. Sure they cast spells, but they don’t do much more. A demon lord seems like he would be more involved and proactive.
It’s not a huge stretch of the imagination to imagine stories of Yeenoghu himself walking the earth in the times before the empires of men, leading a great gnoll warpack.
Zuggtmoy- I like mushrooms as much as the next person, if not more when it comes to pizza, but do they really need their own demon lord? It’s like Pazuzu all over again, only even more pointless. I could see her living in some kind of vast and strange palace of mould and mushrooms where the air is thick with hallucinogenic spores that blur the lines between reality and twisted dreams, but counting them as her domain just seems silly. Can a mushroom really be evil?
Personally, I would re-brand her as some kind of wicked and uncaring fey queen.