|I sometimes wonder if I'd be better off excluding such monsters from my campaign entirely tbh|
they bring some awfully unpleasant implications along with 'em
I could go over individual D&D demons, but frankly, I don't much see the point. They're too intrinsically tied to copyrighted, published settings for me to say anything that a random D&D lore wiki/youtube video won't say just as well, and while they definitely have the occasional neat trick, they rely mainly on 'immune to lots of stuff,' 'lots of spells/psionics' and a poorly explained 'demons summoning demons who can RAW summon more demons' snowball effect that as far as I can tell implies that a single demon should turn into a demon army and take over the world in a matter of hours.
So while there are indeed a great many spiritual servitors of gods that could be subjectively described as 'demons' and make for an extra half-dozen posts, for now I will leave you readers with this post explaining what the term 'demon' actually means in my setting, and a quick rundown on the three major types of demon I actually use with any frequency and then move on to the Displacer Beast. (Oh, you wanted a Dinosaur entry? Dinosaurs were the Reptile Empire before it got divided into Snakes (the faithful of Yg) and Reptiles (those who rejected Yg's wisdom in favor of legs.) And that's all you get, it was really too long ago to care much about.)
First off, Demons & Devils are very nearly almost the same thing as each other and Angels. The whole lot of them are non-physical beings of light created by some big time spirit(often called a 'god'). The difference is that servitors you like are 'angels' and those you don't are 'devils.' Demons are servitors who survived the death of their god and are now independent, either sustaining their light via the willing assistance of mortals who still hold faith, or going the route of Yg-A, the Dragon-Sun, and igniting their souls into something fiery and predatory that can take what it needs by destroying the physical, rather than relying on the wishes and prayers of other beings to maintain their ability to exist in/affect the waking world. Also of note is that a human soul can serve similar roles to a servitor as well, though as they exist independently of a god, they are typically called saints instead, and a 'rogue' human soul with no divine association is referred to as a ghost.
Another thing to remember is that spirit-being taxonomy is incredibly difficult and obfuscated due to centuries of misinformation campaigns from all parties involved. 200 years ago, the leading human scholar on the matter was educated entirely on Alf propaganda and thought foocubi and nymphs were the same thing, the dwarves got all their information from a Yg-A demon and a order-over-truth Sun God, and nobody understood the difference between a nightmare realm and human-unfriendly afterlife.
But I'm sure this time the scholars got everything 100% correct.
The most numerous type of demon, typically no larger than a human child, and typically no smaller than a hand-span. They require very little 'spiritual fuel' so to speak, and as such are fairly easy to bind into contracts that help them preserve their life and power, and are relatively common as familiars and conveyors of spells that were once minor religious miracles, but are now simply ruled over by whimsy.
Imps range from 1-5 hd, and typically either can effect miracles themselves (such as casting Floating Disc by carrying things manually or ethereally) or have, over the past hundreds of years, gotten on good terms with spellwisps themselves. Imps arose from every half-baked wanna-be god and cult leader in the bad old days, and as such are an extremely variable bunch who range from the ridiculous (Imps that eat sleeper's toe-jam) to the horrible (imps that pose as and then eat unwanted children). Imps can be mistaken as mutated halflings/children/fairies in bad light, but their caricature faces, warped forms, and centuries old linguistic tics tend to identify them for what they are.
Standard Spell List as Imps (aka Cherubs or Saints for live religions)
Cure Light Wounds & Other Healing Spells- Imps stitch up the target with needles and call it a day, mostly. Tiny fingers makes them able to do precise surgery for nerves and blood vessels too. Centuries of medical knowledge give 'em good odds of being able to fix poisoning, blindness, etc as well, and in the case of diseases and lesser curses, a swarm of imps driving off the offending spellwisp, proto-soul, or whatever. Some big time curses(think 'sleep till awakened by true loves kiss' tier) are not impressed by flurries of imps however, and can only be beaten by playing their own games.
Detect (Whatever)- Basically any Imp can handle this by flying around in a coterminous realm, peering between worlds, and reporting back on what they perceived. These spells are often single-word affairs, a word like 'elf' or 'undead' or 'magic,' vestigial remnants of very, very dead languages for very, very old imps. These ancient imps appear as winged eyeballs, and are suspected to be the imps of a long-dead bat-god, explaining their lack of anthropomorphism.
Light- another easy to pull off trick, all an imp needs for this is the ghost of a dead candle, torch or lamp that was destroyed before it could ever shed light, and so has unfinished business. The reversal, 'darkness' is also easy- any 'darkness' spell is almost certainly just a halfassed portal to the netherworld that light pours into, to be devoured by the Imp and/or any other spirit-being hoping to bolster the light of their soul with stolen energy.
For people like heleognostics for which light loss is unacceptable, thick burning smoke from a real, non-ghost torch that fell between realms is the typical darkness substitute.
Protection from (Whatever)- This is simply an imp working double-time to deflect unwanted things from coming into contact with the warded targets. Arrows are easy, though only some imps are fast enough to knock them from the sky. Undead are easy because the imp can usually possess part of their body and prevent physical contact, Darkspawn are easy because they are naturally inclined to either avoid light, in which case a glowing imp-soul wards them off, or because they are inclined to chase light, in which case the imp functions as live bait. Other spirit beings and spell wards can be warded off by the sealing of the warded person/area's soul into a pocket realm where it can interact with the physical realm and body, but is inviolable to uninvited guests. (The latter trick is relatively easy for even untrained individuals and is often expressed via saving throws.)
Bless/Curse- Imps making micro-adjustments to stance, arrow direction, and so on, for the combat applications. Otherwise, a very nebulous way of telling an imp to watch over a person/thing and try to serve as a guardian spirit/malicious haunt, with inconsistent results.
Floating Disc/Hold Person/Levitate/Flight/Telekinesis- Just a bunch of imps working together to exert physical force on the waking world from their coterminous realms. Different effects are largely a case of being able to speak multiple dead languages fluently enough to get imps/ghosts/etc from all manner of dead civilizations to work in concert for mightier effects.
Charm Animal/Person/Monster- Ranges from possession, to manipulation via whispers in the ear(false promises of desires granted), or interfering with the brain in a variety of ways that range from so subtle that the target spirit is unaware their meat-puppet has been hacked, to creating a hallucinatory world where the spirit knows something is wrong but can't help but be fooled.
Sleep- imp with ghost-chloroform or a blackjack applied to the astral side of the head.
Magic Missile- Imp with very pointy horns, applied interdimensionally straight through armor.
And so on and so forth, with rather straightforward approaches compared to the reality-bending abilities of more standard eldritch spellwisps.
Those wizards who use demons instead of spellwisps are oft called demonologists, and this method has a not undeserved reputation for being an easy route to power. After all, demons tend to have social circles and can introduce you to more spells, and though they may enjoy things like live sacrifices, they are still psychologically more human than the average spellwisp and so easier to manipulate... though that goes both ways.