Fiends, for those of you who haven't really read the Monster Manual that closely, is the catch-all term for demons, devils, and beings of pure evil in D&D. The cosmos in D&D is built around its alignment system, and so there's a firm distinction between the lawful evil devils and chaotic evil demons.
I actually think that DMs should pay attention to this: it's one of the really interesting quirks of D&D's monsters, and in the lore, devils and demons are at eternal war, with the neutral evil (though I guess lawful-leaning given that they're from Gehenna) Yugoloths working as mercenaries for both sides. Yugoloths were originally called Daemons. In the early 90s, TSR (the previous company to own the rights to D&D) renamed these three categories in an effort to distance the game from accusations by the Religious Right of encouraging Satanism, calling Devils Baatezu, Demons Tanar'ri, and Daemons Yugoloths. When the Satanic Panic subsided and the general culture recognized that portrayal of such things didn't mean endorsing them, they brought back the more familiar names from folklore and myth, but Daemons were probably too similar to Demons in name (they're really the same word) so they stuck with this name.
There are also plenty of other fiend types. At least in 5E, they haven't really fleshed out the fiendish inhabitants of the other Lower Planes, though we did get the three kinds of Demondand from Gehenna. Most fiends belong to the devil, demon, and (in distant third) Yugoloth category, but there are also plenty of uncategorized fiends like Succubi/Incubi, Rakshasas (which are really interesting because they're from the Nine Hells just like devils, but somehow aren't part of that hierarchy,) and Night Hags, as well as some newly-inducted fiends as of 2025 of the prime material plane, Sahuagin and Gnolls.
As a DM, how do we want to use these?
One thing to start off with is that fiends are universally evil. I think the only possible exception is things like the Fiendish Spirit from the Summon Fiend spell, which, like all such summoned beings, is Neutral. Now, I think that there's some room to play with this - if there can be fallen Celestials who turn evil, oughtn't there to be some fiends who have turned to good? However, with some changes in 2025, it does look like Celestials can be Good or Neutral, but what used to be an evil Celestial, such as some Empyreans in 2014, are now categorized as Fiends.
So, I think you could reasonably think of Celestials and Fiends as almost the same creature type - just planar outsiders (though that leaves creatures like Sahuagin or Gnolls in a funny position) but when they're evil, they're fiends.
The point, though, is that if you want players to face foes that they don't have to feel remotely conflicted about fighting and killing, Fiends are arguably the best options: unlike a monster that is evil, where evil is an adjective, fiends are evil in the sense of evil as a noun: they are made of the substance and idea of evil.
You're going to find fiends at every challenge rating. A 1st level party can easily fight Lemures, Spined Devils or Manes, Dretches, etc. And a demon lord like Demogorgon or an archdevil like Zariel can make a perfectly good campaign final boss, with tons of monsters that can fill in the gaps between.
A fight against fiends as a kind of evil army is totally valid. Simultaneously, an evil cult or other sort of enemy faction can have a fiend bound to their service in some way that acts as a powerful living weapon to use against their foes - either as a boss-like encounter, as cannon fodder, or maybe a powerful lieutenant to a mortal villain.
Fiends tend to have pretty powerful stat blocks, but I think if you use them as just monsters to stick in a dungeon room, you're not getting your money's worth out of them.
First off, I would say to look at the Monster Manual (the new one) and how it describes what kind of evil that each fiend represents. Not all of them have such evocative descriptions, but, for example, the Glabrezu, a good mid-CR demon, is described as embodying delusion and predatory guile.
I think you can characterize the demon itself as acting in this way, but you can also associate them with an NPC that holds these dark elements in them.
Fiends are generally corporeal entities (with a handful of exceptions, like Shadow Demons) that might be assumed to move and interact in the world like any other creature. But I think there's a real potential in treating them differently:
In cultures across the world, there is a belief in invisible spirits that might be benevolent or malevolent. "Unclean," wicked, evil spirits have been blamed for malicious behavior, illness, and misfortune.
Among the most popular horror movies of all time is The Exorcist, in which a demon possesses a little girl, and a pair of priests perform an exorcism to try to free the girl from her possession. The most disturbing aspect of the movie is the horrible way that the girl behaves under the influence of the demon (something that a 1970s audience was scandalized by, which only enhanced the legend of the movie).
But it creates a really interesting tension, because the demon cannot be dealt with in some straightforward manner - the body it inhabits is an innocent, after all.
Let's talk about planes:
In a certain way, planes in D&D are just other worlds - other universes, yes, but ones to which you can travel and kind of go about your business as you would on a normal world, even if the sights you see there might be surreal and alien.
Most fantasy (and some science-) fiction that concerns other planes view this in a more spiritual sense - your mind might ascend to some higher plane of existence, but merely being on that other plane means a different kind of existence beyond the physical.
I have some thoughts about the "depths" of planes that I'll probably write about in another post, but while a plane like the Abyss ought to feel very distant to someone on the prime material plane (or one should hope it is,) I think that the fact that there's no physical spatial relationship means that it can be both near and far at the same time.
We often think of a demon possessing someone in fiction in the following way: the demon, if it has a physical form at all, forgoes manifesting that physical form in favor of becoming a non-physical presence within a host.
Oddly, the only demon I can think of that possesses someone in 5E is the Dybbuk, and it only possesses corpses (as someone of half-Jewish ancestry, I love getting bits of Jewish folkore in my fantasy. I have a strong affinity for Golems, as well).
But we need not be limited by stat blocks - your Lich can for sure cast the occasional Meteor Swarm even if it's not in the stat block.
And if we want to give demons and other fiends the ability to possess people - or places - we can think of the mechanics.
(As a side note, I think giving all sorts of fiends the ability to possess targets is fine, but it might be a fun thing to give to demons specifically, as I feel like devils honestly get a little more of a spotlight in 5E and have cool ideas like Soul Coins and all their contracts.)
Now, what is this about possessing a place?
As I often do, I'm thinking of a part of Stephen King's Dark Tower series. In the first book, The Gunslinger, the eponymous hero (really very much an antihero in this volume) finds a building out in the desert that is the lair of a "Speaking Demon," which he is able to force into giving him information by finding the hidden demon jawbone in the walls. The demon can speak within the basement, but has no physical or visible form.
I think the lore mechanics for demonic possession could work the following way: while possessing someone or something, the demon itself is still in the Abyss. Demons are such an infectious presence in the D&D multiverse that summoning them is both risky and difficult. I think the likely reason a demon would try to possess something on the prime material plane would be to get a foothold there.
Now, in terms of in-game mechanics, this is pretty simple: the demon isn't a creature you can fight. If they're possessing a creature, that creature might act in a way directed by the demon, but this could also manifest in varied ways:
Most straightforwardly, a possessed person might be puppeteered by the demon, every action under the demon's control. They might attempt to cause their host to act normally to avoid suspicion.
The other options is that possession isn't direct control, but a subtle influence and even torment. A demon possessing a creature might cause them to see things that aren't there, or even take over the body from time to time and push the host's consciousness into the depths of their mind so that they lose time and wake up having done things they don't remember doing.
Now, this opens up a lot of options for you:
First, it can be tricky as a DM to let a cool villain develop, because players will often try to strike at them as quickly as possible, even trying to cut off their villain monologues in the hopes of getting off an attack before initiative is rolled (DMs, by the way: if the party is just trying to yell out "I cast Disintegrate" before you can say roll initiative without being hidden or taking the villain by surprise, you can tell them they can do that on their turn). If the demon isn't actually physically there for them to attack, you don't need to worry about losing your bad guy.
The other thing it opens up is having a really powerful demon showing up early in a campaign, before the party would be capable of facing them. Just as the possession prevents the party from killing your demon, so too does it prevent your demon from killing the party.
While you can certainly make fun villainous NPCs of any kind of demon, you could even have the possessing demon be some powerful demon lord. Given that they're not physically there, it's quite easy to make a mystery out of what demon it is the party is dealing with. And you can have fun foreshadowing things by having oblique references made to, say, Grazz't.
It's also a way to have recurring fights against the villain: a possession might be thwarted by killing the host, or by using spells like Dispel Evil and Good. But each defeat or casting out of the demon only means delaying things. Certainly, the demonic restoration feature in 2025's Monster Manual demons already gives you something of this functionality - killing them in any other plane just sends them back to the Abyss.
One way you could play this, actually, is that a possessed creature gradually (or perhaps very suddenly) transforms into the fiend, using their stat block and describing them as taking on the fiendish appearance. But easily enough, you could have a demon possessing your Bandit Captains, your Mages, or even a non-humanoid creature like a particularly vicious Hill Giant or oddly cunning Bulette.
