Not so much the good guys as the less evil guys. God himself is mostly absent.
Except towards the end, where he shows himself to be evil.
The forces of Hell and Heaven seem more like rival sports teams than vicious enemies in Ah! My Goddess, but even so the demons can be nasty.
The DiC dub of Sailor Moon lumped all of the show's villains and monsters from the first two seasons into a single evil force called the "Negaverse".
In the original manga, they were all forms of the God of Evil Chaos.
In Bleach, Hell is not among the villains. In one episode, the Soul Reapers defeat a fallen ghost who was also a serial killer during his life. A Soul Reaper cannot purify sins made by the living, so the Gate of Hell opens up and a demon reaches out and grabs the damned soul.
Much better in the manga where the giant demon impales him on an equally giant sword.
On the other hand, Hueco Mundo ("Hollow World") is a poster child of this trope. The Hollows themselves were the early antagonists, and the series's Big Bad eventually sets up shop there.
In the latest film we meet the "Togabito" (defilers), who are the humanoid denizens of Hell.
The dead villains from Dragon Ball and Dragon Ball Z were shown in hell from time to time. Due to censorship issues at the time, any time in the dub the word "HELL" was on a uniform or on the wall, it was edited to "HFIL" or "Home For Infinite Losers." This is the subject of amusement and/or eyerolling among fans.
The demons of Berserk are quite powerful, diverse and horrific. Every one of them were once humans who got hold of a Behelit and sold their souls and sacrificed those closest to them to the demonic gods of the Godhand in exchange for being reborn as demons. The Godhand themselves are servants of the Idea of Evil, a quite evil God that governs the Berserk universe and manipulates events so that behelits get passed down to those destined to use them so that more demons and members of the Godhand get created. Needless to say, Berserk's world is every bit of a crappy place to live as the Warhammer 40000 universe.
And, with the existence of Feudal worlds in 40k, it's quite easy to WMG that they're the same 'verse.
No two Orphans from Mai-HiME are alike, and their presence is made even more eerie once the characters (and the viewers) realize where (and who) they're coming from.
The Hundred Demon Empire of Getter Robo G. The Dragon of the organization looks like Hitler with demon horns. Like a bonus, his name is "Hidler".
Pulled off in the 10th episode of Kanokon, when a horde of them attack the school. When he reaches his Five-Tailed Form, Kouta lets loose an expanding energy wave that completely obliterates the horde of baddies (save their leader, who flees).
Even more thoroughly subverted by Hyper Police, in which the invasion is long over and the demons are a normal part of the landscape (to the point where humans are a rare and protected species).
Mahou Sensei Negima!: Negi's hometown was doomed by one of those. Is not clear if they do come from Hell, but they are called demons anyway.
Then later on, Dynamis summons an army of several hundred thousand demons. Same as above, it's not stated whether they're actually from hell, though.
Just a bit later, we are (re)introduced to Zazie Rainyday's sister Poyo, and Zazie too. They are heavily implied to be from hell.
The Dark Liege Army in Nora. Subverted in the fact that they're actually the good guys.
Shows up from time to time in The DCU. Teen Titans has had to deal with Trigon, Superman has traded blows with Karkull, and even Batman has fought both alongside and against Etrigan the Rhyming Demon. The Sandman makes quite a few visits there as well.
Mephisto and his son Blackheart serve as the rulers of a Hell-like dimension in the Marvel Universe. They are responsible for the creation of any number of heroes and villains, such as Ghost Rider (who sold his soul) and Dr. Doom (who tried to access Mephisto's dimension to save his mother's soul, but wound up scarring his own face). Blackheart was the primary villain of the Ghost Rider movie, with bookend appearances by Mephistopheles.
Marvel has been largely inconsistent in its portrayal of hell (largely due, it seems, to an unwillingness to flatly confirm or deny the existence of God in the comics, quite unlike DC). The current Ghost Rider series had Satan himself (not Mephisto, Satannish or any other almost-Satan Marvel has used in the past). For the most part, Marvel seems to go with the idea that there are many different hells, with many different devils.
Originally Johnny Blaze had made a deal with the same Satan that was the father of Daimon Hellstrom (Son of Satan, natch) and Satanna, who definitely was NOT Mephisto.
Note that part of the inconsistency has been explained by the fact that demons lie all the time.
The Neyaphem, a race of exiled demonic mutants led by Azazel, who is also the father of Abyss, Kiwi Black, and Nightcrawler.
In Lucifer a damned but repentent human soul, Christopher Rudd becomes The Messiah of Hell, and leads the demons and the damned together against the Lilim attempting to conquer the Silver City...and conquer it themselves as revenge for the injustice of allowing Hell to exist.
In Army of Darkness, a bunch of dead bodies are possessed by demons released by the Necromonicon Ex Mortis. They attack a medieval castle, and it's up to Ash, his chainsaw, and his boomstick to defeat them.
The Gate has some kids accidentally summon demons in their backyard by contrived coincidence.
In the Clive Barker novella "The Hellbound Heart", the Film of the BookHellraiser and an Expanded Universe based on the latter, the Legions of Hell are the Cenobites (a word originally meaning simply monks in monastic orders, as distinct from "eremites" or hermit monks). Hell is not a place for punishment of sins as such: souls are lured there simply by the temptation of solving puzzles.
The "feeders" from Terry Brooks' The Knight of the Word novels almost fit the trope. Though they're dark and oozy and scary, they aren't very dangerous; they're more harbingers of evil than evil itself.
There are actual demons in that series, the servants of a being called the Void, but there aren't really enough of them to call a "legion" (and they seem to have a hard time working with each other anyway). The highest number of demons ever seen together was four, and that was an explicitly unusual case.
Demons swarmed out of the Darkwood in Simon R. Green's Blue Moon Rising. In Beyond the Blue Moon, it turned out that the demons were really humans transformed into murderous monsters, subverting this trope.
Paradise Lost features the legions named in a device called an "epic catalogue". Usually used to name the various heroes on the quest (see The Iliad for a classic example), this version instead mentions the various gods of other religions who form the forces of Satan.
The Belgariad and The Malloreon feature demons as a one-shot problem in the former, and a serious threat in the latter. The series makes a point of stating that even a normal Demon can't be fought without sorcery, another Demon, or the aid of a God; one of the antagonists is able to usurp control of an entire country just by threatening to unleash his hordes of Demons. Naturally the two Demon Lords who put in an appearance later are just shy of indestructible.
The Dresden Files delivers in its usual fantasy-kitchen-sink fashion. Demons, Fallen Angels, the Devil, and all other shades of Hell exist, alongside all of Heaven, and the Nevernever, with the Fae, whos Blue and Orange Morality isn't evil per se but is often fairly destructive.
Earlier in the series, Demons had more of a presence in the story, temptations and all, but their place was usurped by the Fae. Lately, only the "Nickleheads" (Fallen Angels with mortal hosts) have made much of a presence for the Down-Below team.
A big point of the series is that while Good and Evil are real, a lot more to life takes place in the grey, orange, and blue. Being a good person is about always working hard at doing the right thing and being clever about it, not necessarily fighting those Legions of Hell. That said, good, strong people also fight Evil. It's just one of the things they do.
The Legions of Hell feature prominently in the Riftwar Cycle, particularly the Serpentwar and Demonwar subseries. Essentially, this setting has reality in metaphysical layers, with each layer being more dangerous than the one above it- any layers below yours can be considered hell (and yes, this means that the mortal world is considered part of hell by angels, who come from the upper levels). Demons come from the fourth and fifth layers of reality below the mortal world, and have repeatedly tried to conquer it in order to feed on the life force of its inhabitants. There's also the Dasati, from the layer immediately below ours, who aren't demons per se but as an Always Chaotic EvilProud Warrior Race they're still plenty nasty and their reality is the first level of hell from a human perspective. On the levels belowThe Legions of Hell you start getting reallybad things.
The Jon Shannow book Wolf in Shadow by David Gemmell has the Hellborn. Even though they are humans, they are Satan-worshippers and explicitly model themselves after this trope.
Power Rangers Samurai has the Nighloks, who come from the Sanzu River (the Japanese idea of the River Styx).
In addition, Power Rangers Ninja Storm had a similar principle, where Lothor's monsters were sent to the Abyss of Evil when destroyed. Lothor's plan was to have the Rangers fill the Abyss until it burst, causing an en masse monster resurrection.
Supernatural had the legions of hell bust loose at the end of season two.
They fight them for the rest of the series, on and off, though not as much after defeating Satan at the end of Season Five, and less still in Seven, largely because they were so nerfed . Offscreen Moment of Awesome when the angels (with Castiel apparently taking point, or at least the last survivor of those who took point) laid siege to Hell and cut their way in to rescue Dean, allowing him to make Like a Badass out of Hell without giving in to Power Creep much. (Dean actually took a hit to his power after Hell due to PTSD, though in the Bad Future he was using his education there for Jack Bauer Interrogation Technique.)
Sam had Power Creep that season, but it was a badthing. Cas had it in season six. It was a bad thing again. Supernatural is strict about these matters.
There is subtle creep, in that in early seasons they could barely handle a single demon, while late in six Dean takes a whole series prisoner in the attempt to torture info out of them. The last one flattens him once it gets out due to overconfident carelessness, though, so it's really all a matter of refinement of technique due to practice.
Seen in Fireaxe's Food For The Gods, although in this case the army was composed not only of demons, but of damned souls who were very pissed off about where they'd ended up.
Similarly, the Spectres fulfill this role in Wraith The Oblivion, with the Malfeans being the overlords waiting til they get to eat reality. The various servants of the Wyrm in Werewolf The Apocalypse might also count.
The New World Of Darkness has, as of Inferno, introduced Hell into the setting. Hell plays host to a number of demons that are born of the first fleeting moments of human wickedness, who occasionally come to humans and offer them great power for a little price... For extra fun, ghosts and 'regular' spirits can be corrupted into demons too.
A new gameline, so far known as Demon:the ??? is scheduled to come out late 2013. Presumably it will be related to Hell somehow.
One of the main factions in Warhammer 40000 is Chaos, spewing forth from the Eye of Terror: a rift in spacetime that allows access to the Warp, a nightmare realm made of the emotions and thoughts of the entirety of sapient life in the galaxy.
Chaos comes in three main flavors. You have your standard daemons, entities formed from the aforementioned thoughts and emotions of mortals who are usually aligned with one of the main four ChaosGods. You have legions of power armor-wearing Super Soldiers, sometimes bearing some nasty mutations and "gifts" from the gods, each led by an immortal Daemon Prince. Finally you have the more subtle angle; cults disguised as innocuous organizations, daemonic possession of unidentified psykers, and the manipulation of Genre Blind individuals. Hilarity Ensues.
Don't forget the actual Legion of the Damned, who are the ghosts of dead space marines, that show up on the battlefield to aid fellow Marines when the need is great. They sport armor emblazoned with flames and scorched bones, and they are described as being like smoky shadows in appearance, flickering like a flame in the wind as they plod across the battlefield.
Dungeons & Dragons draws a distinction between Devils, who are Lawful Evil and Demons, who are Chaotic Evil. The former are a race of Chessmasters who seek to conquer the universe, where as the later are out to destroy all of creation. Each has their own home plane: The Nine Hells of Baator for the devils, and The infinite Layers of the Abyss for the demons. Both races have armies of astronomical size, but fortunately for the rest of creation, they occupy most of their time fighting each other in an eternal conflict called the Blood War.
Devils even include "Legion Devils"... the literal Legions of Hell.
Don't forget the Neutral Evil Daemons, although the developers of the new edition seem to have done so.
Yugoloths fit best with the Demons, so they were all shipped to the Abyss in 4th Ed.
Like the WoD example above, another Tabletop Game, In Nomine, gave players a chance to enact this trope as demons operating on Earth. Unlike the above example, however, it gave an equal chance to fight for the other side, too.
Well, not so equal, because Angels have higher stats, and a military organization to support them. Demons have to work undercover and are more weak. The reason? All of this is a Game, and God is a cheater...
Exalted has a really bizarre hell. It's ruled over by the Yozis (who created the world and ruled over it until the Exalted deposed them), and the Can keeping them Sealed is the inside-out body of the mightiest of their number. Each Yozi has a large number of souls, which are the immensely powerful Third Circle Demons. Those have souls which are the Second Circle Demons, and all of the above created the hordes of Mooks that are First Circle Demons.
Exalted also has the Underworld, the inverted shadow of Creation created when the Exalted armies killed some of the Primordials, the beings that created the universe. Killing beings who could not die caused so much chaos that the Exalted decided to seal the rest of the Primordials away instead of killing them, turning them into the Yozis. Because the Primordials were outside the limits of death and time, however, they didn't actually die but instead became creatures now known as the Neverborn, intent on ending their existence by sending their own legions of the dead to take the rest of Creation down with them.
Infernum is a third-party setting that uses the 3.5 rules for Dungeons & Dragons and, as the name suggests, this trope is all over it. In fact, the default assumption is that the party members are demons. It has heavy roots in Christian beliefs, mainly Dante's Inferno, but is twisted and changed into its own unique setting. For a start, the demons are the result of vile crossbreeding experiments conducted between rebellious angels and "spawn" (prototypes of earthly lifeforms) in an effort to breed warrior-slaves... only for the demons to decide they didn't like the idea of being cannon fodder and promptly devour every last one of their "fathers" that didn't run for their life clean out of reality. Many demons at least profess not to believe in Heaven, and almost none believe that it's anything like the humans think it is (the Fallen Angels can't comment, having forgotten everything down to the reason why they Fell in the first place). There's also vague hints of even stranger forces in the multiverse; Benandanti are humans "touched" by nature spirits, whose souls travel to Hell in the guise of werewolves to steal souls to restore the vitality of nature, while Brokenlanders are the ghostly remnants of Quilipoth, another universe so ancient there's nothing left but a single ringworld orbiting the last dying cinder of a star.
The current story in Rifts is the "Minion War", a war between two separate Legions of Hell: The Demon of Hades, and the Deevils of Dyval. It's beginning to spill over into other dimensions.
Magic: The Gathering has a variety of demons and similar events. The best known legions scenario was in the Invasion block, which had the biomechanical horrors of Phyrexia invading the plane of Dominaria.
a Native American name; Native Americans are generally prominent in the setting
are responsible for creating... well, technically every single ghoulie and monster in the setting. They also fuel the spells of Hucksters, are the ultimate cause behind the Science-Related Memetic Disorder of the Mad Scientist "class", and are the secret source behind the Super Fuel that drives the Cattle Punk of the setting. The manitou's efforts occasionally bite them on the ass; most prominently, while many corpses possessed by a manitou rise up as Walkin' Dead, a rare few instead become the Harrowed, which are intelligent zombies, with the manitou constantly struggling with the original personality for control over the body, who can and often do use their supernatural powers to battle the manitou's purpose.
Doom and its sequels involve an invasion from Hell. While many many video games (eg Quake, Half-Life) are based around monsters pouring forth from another realm, Doom is one of the few to go the whole hog and use Hell itself (although Satan himself is conspicuously absent).
Both the contemporary novelization and the later movie avoided this trope, by using aliens and genetic engineering respectively.
The Diablo video games are about the Legions of Hell attempting the world's destruction. While in the first part you venture ever deeper into ever more nightmarish caverns, in the second game the last act is in Hell, and in the third game, the Legions of Hell storm Heaven itself, and you have to go to Hell to shut down the gateways they're using to invade. The expansion to the second game is about an attempt to graft Hell and the mortal world together, and there are portals to Hell where you can go and loot stuff.
Mehrunes Dagon's Daedra & The Dremora (his favorite servants) are much like this due to their innate destructive impulses. Most lesser Daedra (and what passes for 'animals' in the daedric planes) are hostile to the majority of mortal peoples and wreak major havoc in Tamriel unless their leading Prince restrains them. The Princes themselves largely follow Blue and Orange Morality (occasionally happening to align with human virtues) and therefore do not fall completely under this trope unless they're in the mood for some genocide.
The Burning Legion in the Warcraft series, an army of demons whose goal is to unmake the universe. They scour all life from the planets they conquer, and the only beings they spare are those whom their leaders deem fit to be "recruited", corrupted and pressed into their crusade.
A important chunck of backstory for all Ogre Battle games was once that the Underworld's armies of Demons, Ogres and other such nasties invaded Earth, who were aided by Heaven's armies in the titular "Ogre Battle". The Underworld lost, but parts of the legions can still be summoned by humans. They are often very important plot point, like in Ogre Battle March Of The Black Queen, where Rashidi contacts a General of said Legion, Galf or in Ogre Battle 64 is a bunch of Ogres being summoned by The Holy Lodis Empire. However, in battle, you can persuade some of them to join you!
The Heartless of Kingdom Hearts are, as that trope describes, The Virus made from the 'darkness' in human hearts. The original "Pureblood" Heartless are entirely Made of Evil and have been around for a long time, but it wasn't until a Mad Scientist started messing with them that they became the world-devouring threat they were by the first game. The sequel adds gray-bodied Nobodies, the cast-off shells of Heartless victims.
The Ing of Metroid Prime 2 are made reminiscent of this. They are just transdimensional beeings though. They possess creatures and even come from Dark Aether, a dimension that kills any "light creature" (anything that is not Ing) almost insantly.
The Darkspawn of Dragon Age: Origins are a kind of plague Orcs. The Chantry's version of the Darkspawn taint's origin makes the Darkspawn seem demonic - they were overambitious mages who tried to physically break into Dream Land and find the mysterious Golden City at its heart, which the Chantry believes to be the home of the setting's God. It spat them back out as twisted monsters who started a centuries-long cycle of pain. The sequel's Legacy DLC reveals there's some truth to this, although the City may have been "corrupt" before they got there.
The game does feature actual demons as well, which are evil spirits from the Fade that possess the living or dead bodies of human beings, usually a mage due to their ability to go to the Fade in the first place, and feed upon the psychic energies of living beings. There are five known ranks of demons; Rage, Hunger, Sloth, Desire and Pride, in order both of strength and intelligence of the complexity of the emotion they are feeding on. Destroying the host only sends the demon back to whence it came unharmed, and though some demons are able to manifest in the physical world alone, killing them seems to have the same effect. Their hosts usually mutate when possessed into a Humanoid Abomination of varying degrees of Body Horror. Demons are rarer than Darkspawn, but are broadly smarter, stronger and more dangerous.
The Legions of the Damned are one of the playable factions in the Disciples turn-based strategy series. Their leader, Bethrezen, is given a somewhat sympathetic backstory, but the Legion's Mooks are unquestionably the nastiest faction in the game.
The Bydo in R-Type are arguably this, even though they were created by humanity. Thing is, they were locked into a pocket dimension (read: the future Hell) after they proved to have Gone Horribly Right, and they were able to use it as a staging ground to attack humanity several centuries into the past. Now consider that there isn't all that much reason—at least, nothing mentioned in-game—to believe they would have restricted themselves to just the recent past...
In Bujingai, the earth is overrun with demons of various kind. It's implied that their mooks (seen as golem-like warriors with seals and swords) were once humans turned into monsters by a strange radiation.
The Kreegan infestation in Might & Magic is (in-universe) commonly thought to be this (especially played up in Heroes of Might and Magic III). As it turns out, they are actually alien invaders that sweep across the Galaxy as a as a plague of locust... and happen to look like the common myths of devils and demons (or possibly have breeds that look like the actual devils and demons).
Demons are trying to open a gateway into the world in Planes Of Eldlor. Seeing as they are described as having an army at the ready, they are probably not looking to enter peacefully.
These used to exist in Dominic Deegan, until Karnak blew up Hell. No word on whether any survived Hell's destruction and Karnak's subsequent self-coronation.
The "demonic hordes" are the main antagonist of The Senkari, although it's not clear whether they're really evil or just on the other side.
Somewhat subverted in The Salvation War, where the demons are apparently genetic offshoots from human ancestors — and are completely mortal. Superhuman strength, speed (on foot at least), some with wings, and not a few with powers, but all killable (AND HOW!). They only managed to successfully invade mostly empty desert before being pushed back into Hell and then being invaded by HUMANS. Heck, a human kills Asmodeus with multiple sniper rifle bullets to the head, and there's nothing showing that Satan isn't mortal too. In fact, he really only survives a recent attempt by the humans to assassinate him by bombing his palace by sheer luck; he just happened to be out of the city at the time. The angels appear to be slightly tougher, and much faster, but by no means immortal themselves.
Satan was killed when he took two anti-ship missiles to the face, although he would have actually survived and possibly recovered from the first.
They may be killable, but they seem to live pretty much forever if they avoid a violent death.
In the South Park episode "Best Friends Forever," the forces of heaven fought back the forces of hell with the help of Kenny and a divine Sony PSP. The legions themselves owed much design- and action-wise to the Orcs from Peter Jackson's Lord of the Rings films.
The setting of Jimmy Two-Shoes is heavily implied to be Hell, and in the original pitch it officially was. The citizens are just one of the reasons why its obvious it's Hell, with all the monsters and demons.
In Futurama: The Beast With a Billion Backs, Bender recruits an Army of the Damned from the Robot Devil to take over Earth. After every living being in the universe decides to move in with Yivo instead, he leads the army in a pirate-themedattack on Yivo Shklerself.
In Wakfu, the Sushu armies certainly fit the description, a horde of evil beings who seek only to destroy anything good or beautiful.