"Well, like any religion, the beliefs can tend toward positive or negative ends — can be used for good or evil. Christianity, for example, has its Doppelgänger, Satanism. Any time you attempt to set up an icon to explain evil, you invite some warped mind to worship it."
Most often a Fantasy trope, the Religion of Evil has no pretenses of being anything other than... well, evil. Quite often it'll be a Card-Carrying Villain that refers to itself as evil; sometimes it won't say that word straight out, but its tenets and actions will be such that its followers are necessarily evil. Any time a temple's decoration involves lanterns made out of the skulls of their Human Sacrifices, it probably qualifies here (but see Sedlec Ossuary for a counter-example).
This religion has three common forms:
The Evil Counterpart to a good religion, whose deity is not so much worshiped or prayed for rather than appeased and bribed towards leaving mortals alone.
In the latter cases, most of the time the masses will follow the religion out of fear rather than any genuine religious devotion. In the rarer cases when it exists openly in a "good" nation, it tends to be treated as a legitimate minority faith, perhaps worshiping the evil members of the pantheon. Members of the Deadly Decadent Court may attend, either in search of power or just for the thrill.
Because Religion is Magic, devout practitioners and high ranking clergy will have powers, magic and other forms of The Dark Arts. You can expect the High Priest to emanate a Cross-Melting Aura.
This trope is the polar opposite of the Saintly Church. Also contrast with Path of Inspiration, where an evil religion masquerades as a more ordinary faith. If a genuinely "good" religion is being twisted into evil, it's the Corrupt Church. Whatever the religion of evil worships will generally be the Bigger Bad, with demon lords, archdevils, Eldritch Abominations, gods of evil and Satan himself all being likely candidates.
Media portrayals of Satanism tend to fall directly into this trope; for this, see Hollywood Satanism.
No Real Life examples, please! This is a very controversial subject and likely to invoke edit wars. It is sufficient to say that certain features of the Religion of Evil can be found in historical religions of human history.
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Naruto has Hidan, a follower of Jashin ("evil god"). Aside from the lengthy praying and kickass immortality jutsu, he's required to kill his opponents via a blood-drinking, masochistic ritual. The first thing he did when joining the religion was massacre everyone in his peaceful ninja village because he hated peace.
Higurashi no Naku Koro ni's "question arc" revolves around an "Oyashiro-sama" based religion. Partially averted when "Oyashiro" makes a proper appearance and is largely powerless and adorable. As well as very unhappy with the things that people do in her name.
Dragon Ball GT has a religion that worships a giant gargoyle named Luud that turns people into dolls and absorbs their energy. It turns out to be just a giant android.
The Followers of Kira from Death Note. Ironically they are killed off by Kira (actually Teru Mikami, a follower of Kira, but Light would have done it himself if he wasn't being watched) as they were obviously creating the cult simply for prestige and greed.
After Light's death they reemerge, praying for Kira's return. Possibly a subversion since we never see them do anything evil.
The Society of Light in Yu-Gi-Oh! GX is never actually referred to as a cult, but it clearly is. (Its kind of obvious, given that meetings they have are clearly sermons. And seeing as the Light of Ruin is ultimately behind it, clearly evil.)
In Fairy Tail, various cults worship the Black Mage Zeref, even centuries after his supposed death. Dark Guild Tartarus in particular is led by Zeref's strongest demons and worship him as their master.
At least they do it smart, with four branches that focus on four sins: the Deceit wing gets people to learn the religion by claiming that the whole thing is a hoax that they're seeking to expose, the Lust wing runs brothels and either slowly corrupts or outright blackmails repeat clients into Church membership, the Greed wing plays up just how much money there is to be made in crime, and the Murder wing... well, that's for people who already like killing. Hey, sometimes you gotta preach to the choir.
And Yea did the prophet Joker raise the holy crowbar to the heretic Robin's head. And he did anoint him and again until his brains could be seen. And There Was Much Rejoicing.
In the writers' notes for 52 (the series which introduced the Church of Crime), Mark Waid laments that they didn't pick a better name, noting that an evangelist saying, "Come and join the Church of Crime" probably won't get many converts.
The Church of Blood! Even Nightwing was a member at one point! Brainwashing was involved. All Hail Brother Blood!
There is also the Cult of Kobra, which controls a billion-dollar international crime syndicate and seeks to usher in the Kali Yuga (an "age of chaos") via terrorist attacks.
Speaking of Kali, Ravan, a member of the Jihad and later the Suicide Squad, is a member of an extremist Thugee cult with an opposite mission - to delay the coming of Kali Yuga - by killing. Every time he kills, Ravan says "Another thousand years, Kali..."
Another DCU example is the unnamed group of evil-worshipping monks that trained Prometheus and gave him the key to enter an alternate dimension.
Apokolips, where daily life for its billions of enslaved and brutalized denizens revolves around the endless, eternal worship of the resident despotic ruler and god-emperor of tyranny, the cruel and malevolent Darkseid. Some really evil villains from other parts of the cosmos worship him too- including the aforementioned Church of Crime.
Shuma-Gorath is another demonic god-tyrant who is worshipped throughout the cosmos, but while Dormammu is "merely" the ruler of one dimension, Shuma-Gorath is the all-powerful ruler of over a hundred entire universes and is close to being The Omnipotent for all the power he has. He is worshipped by gods of gods and once ruled the Earth in the distant past (twice) where he commanded human sacrifice. A seriously evil Cosmic Horror and one of the most powerful villains in the entire Marvel cosmology.
The Guardians of the Galaxy had a Story Arc that culminated in the Crisis CrossoverThe Thanos Imperative dealing with the Cancerverse and its residents, the Universal Church of Truth, led by the Evil Counterparts of Adam Warlock and Captain Marvell and worshipping the new rulers the Many-Angled Ones, whose members include Shuma-Gorath and its brethern. The Many-Angled Ones tricked Mar-Vell into killing death, procedeed to brainwash the entire universe (except for the Machine Resistance), and added it to their ever growing collection of universes. Eventually, with Death gone, life grew out of control and the Universal Church of Truth invaded the mainstream Marvel-verse and planned to repeat the process, but were ultimately destroyed by the Guardians and Thanos.
Thanos himself is apparently a religious figure in a nihilistic cult he inspired / founded on an alien planet, headquarted in something called the Nietzsche institute. He doesn't bother to tell them when he decided that nihilism wasn't working for him anymore.
Norman Osborn once headed a cult called the Brotherhood of the Scrier, which worshipped a nigh-omnipotent cosmic entity of the same name. The cult is an international rriminal organization, but the Scrier himself is actually closer to a True Neutral and not that bad a guy- it is unclear if they know this, so if they assume he really is evil and worship him anyway then they are this trope, if mistakenly. One faction in particular counts more than most when they followed Osborn after he was kicked out and became the Cult of the Goblin.
In Ultimate X-Men The Hellfire / Phoenix / Shi'Ar religion is this, "saviour" of the universe by cleansing the current one.
The Pride from the Runaways is one of the Seceret Circle of Secerets variety. They sarifice young women to a trio of fallen angels who want to destroy all of humanity.
The Church of Elvon in Nexus believes that all high technology increases energy consumption, and therefore increases entropy and accelerates the heat death of the universe. They therefore preach the overthrow of technological civilization by force. Violent, bloody, sadistic force. How this religion spread across the galaxy, well, hypocrisy is the tribute that virtue pays to vice?
The Kali worshippers in Indiana Jones and the Temple of Doom, very loosely based on the Real Life Thuggee cult. In truth, the Thugs were more like highway bandits, who strangled travelers with scarves in their sleep to rob them. While a cult did develop that gave religious motivations to their actions, it was not very widespread. Far from an evil cult goddess, Kali is the Goddess of eternal energy, the Punisher Of Evil, and is very popular in mainstream Hinduism.
The Seventh Victim is complicated; one of the heroes gives a Shut Up, Hannibal! near the end where he accuses the Satanists of being Nietzsche wannabes. Also, the Satanists are mostly nonviolent and prefer to use social pressure to make people commit suicide rather than just killing them (one of them even comments on the contradiction inherent in their refusal to commit violence versus their need to kill all who betray them). But the leader does claim that he worships evil and refers to himself as "evil".
Waterworld features a Big Bad whose title is "Deacon of the Deep". His minions, known as Smokers to everyone else in Waterworld, are part of the Deacon's "Church of Eternal Growth", on a mission to conquer and consume all they survey.
Dagon (2001): In a small town called Imboca on the coast of Galicia in Spain, a drunken tramp named Ezequiel tells Paul how the denizens of the town have overthrown Christianity in favour of the fish god Dagon, who has brought them wealth from the sea in the form of fish and gold, and the only way to appease the fish god, Dagon was through Human Sacrfice, which they did to Paul's girlfriend Barbara in the end of the film.
The Necromongers in The Chronicles of Riddick. They worship pain and death, have a very sinister gothic design theme, are on a holy crusade to convert the universe to their ways and kill anyone who refuses, and ultimately want to follow their "holy half-dead" Lord Marshal into the underverse.
From the Lone Wolf series, the Acolytes of Vashna and the Cener Druids. The Acolytes wish to resurrect Vashna, the first and most powerful of the Darklords, so that he'd conquer the world with an army of undead. The Druids plan to kill about every living being beside themselves with biological warfare. And the two are allies.
In Kushiel's Avatar by Jacqueline Carey, a Fantasy Counterpart Culture of Persia is tired of worshiping Ahura Mazda and getting nothing for it; a cult that worships his enemy, the evil Angra Mainyu kills the leaders of the country and takes over. It seems to pay off, as the members of this cult, who describe themselves as evil, actually do gain supernatural power, but it scares everyone else, gods and humans alike.
The worship of Torak in the Belgariad, a prime example of the case where only the Grolims (priests) have any real faith and the masses follow out of fear. They keep the faith into the Malloreon, even though Torak is now dead. At the end, it's said that Eriond's first act as a god will be turning the Grolims towards a less vicious path.
The Black Canons in Teresa Edgerton's Celydonn trilogy. They camouflage themselves by taking on the external trappings of whatever religion is currently in power.
The Faith of the Pannion Seer in Steve Erickson's Malazan Book of the Fallen is a particularly nasty example, most evident in its conversion of the combat capable population into cannibal fanatic shock troops (through implied Mind Control) and the rest into their supplies.
The Sisters of the Dark in the Sword of Truth series have it as their stated goal to unleash the Keeper (read: Satan) on the world and end all life. (Except theirs; they believe that they will be granted immortality for doing so.)
Darkfriends from The Wheel of Time worship The Dark One, the antithesis of the Creator who is a Sealed Evil in a Can, the Can in this case being the Pattern, of which the world / universe is the tapestry. If he escapes his prison he will unmake the Pattern and bring about The End of the World as We Know It, but Darkfriends believe that once he has done this he will remake the world in his image and reward his loyal worshippers by resurrecting them as immortals who lord over the rest of humanity. In practice though most are actually quite terrified of the idea that this will happen in their lifetime and the majority simply joined for the perks, since Darkfriends tend to be quite well connected and for most Darkfriends its more like an international, criminal secret society and mutual benefit group. The books happen to take place as the Dark One is starting to escape, and the initial reaction of every Darkfriend is abject terror followed by varying degrees of fanaticism, resentment and paranoia towards non-Darkfriends and each other. Almost every single Darkfriend is both terrified of the Dark One beyond measure and entertains the delusions that they will be his favoured subject, and almost none of them dare to think that the Dark One is simply an Omnicidal Maniac who will just destroy the world and that's it.
Robin Jarvis has three of these in his two Deptford Mice trilogies; the cults of Jupiter, Hobb (and his co-gods Mabb and Bauchan), and Suruth Scarophion. All of them practice sacrifice of their fellow sapient animals, since there are no human characters, in gruesome manners. Jupiter eats his sacrifices, Hobb's unfortunately named followers, the Hobbers skin their victims alive in a process referred to as the "bloodybones", and Scarophion's cult (known as the Scale) poison their victims with his blood, which dissolves the victim into a puddle of tar. You know, for kids.
Then in The Whitby Witches we get the Coven of the Black Sceptre, aka: the Brides of Crozier, a cult of crazy fangirl witches who can turn into dogs and worship the evil warlock Nathaniel Crozier who treats them like scum.
The Mijak religion in Karen Miller's Godspeaker Trilogy is an example of this, contrasting with the religion of Etherea. It's symbol was a scorpion which they bred in the temples to fill in a pool and swim in for divination. Animal blood was used a lot in ritual, drunk hot from the carcass and poured into pools to determine the will of god. Priests and the rulers of Mijak were chastised with beatings, including a young Zandakar. In fact they are not worshipping a god, but a dark power they believe to be God.
The Worshippers of Helgrind from Christopher Paolini's Inheritance Cycle. They worship a mountain that they refer to as their "dread and terrible lord," kill slaves as sacrifices, drink human blood, and make offerings of their own flesh and limbs.
Both played straight and subverted in Mike Carey's post-The Sandman novels.
Played with in the Discworld. There are a few demon-worshipping cults, and Evil Harry Dread has occasionally run a temple of evil, but most people who've compared Discworld demons to Discworld gods have decided the main difference is PR. The worshippers of Bel-Shamharoth, the Soul Eater, on the other hand, are clearly insane. And mostly very, very dead.
It should be pointed out that there is a "Young Men's Reformed Cultists of the Ichor God Bel-Shamharoth Association", or "YMPA" in Ankh-Morpork...
In the Nantucket novels, the sadistic Dr. Alice Hong starts a cult in Bronze-Age Achaea (Greece), with herself as the avatar of the Lady of Pain, to be worshipped by torture and sacrifice. This cult actually has official status within the Sacred Collegium, as Hong is the senior wife and lieutenant of William Walker, King of Men.
The Cult of Malik Nous, the Peacock Angel, in the alternate history novel The Peshawar Lancers. It is the prevalent religion in the remains of tsarist Russia following a meteor shower that destroys most of the northern hemisphere and involves the worship of the Slavic dark god Tchernabog, cannibalism and virgins who can tell the future (to make matters worse, their powers eventually drive them to insanity, at which point they are taken away to become breeders for the next generation).
The Church Universal and Triumphant (C.U.T.) in his Emberverse series.
The Yuuzhan Vong from the Star WarsExpanded Universe have a priesthood devoted to genocide, non-consensual bio-forming, and extreme masochism.
The Thebans in the Starfire books, who embark on a holy crusade to bring humans back into the light of worshipping Holy Terra. Never mind that they're aliens (explained later, trust me). There is seriously one Theban dude who does not like the prospect of the campaign.
In J. R. R. Tolkien's The Lord of the Rings, the Dark Lord Sauron was worshipped as a god-king by the humans under his control. (The orcs, however, just saw him as their cruel slave-driver.) Aside from their leaders, though, they weren't themselves evil — they were forced to worship Sauron out of fear through lies and threats, not faith or devotion. This doesn't really come across in the movies.
Sauron also managed to convince Ar-Pharazôn and the Númenóreans to worship his master Morgoth/Melkor with Human Sacrifice, as part of the chain of events that lead to the downfall of Númenor described in Akallabêth.
The religion of the Pah-Wraiths in the Star Trek: Millennium series may qualify, as the "Ascendants"' principal aim was to destroy the universe for reasons of cosmic harmony. This may also be an example of Utopia Justifies the Means.
The established church in Gene Wolfe's Book of the Long Sun is this, since the gods being worshiped, with one or two exceptions, are deranged sociopaths whose idea of a commandment is "Overthrow your government and let me know when it's done; if you sacrifice enough children you'll probably get my attention." Interestingly, though, the church itself is closer to a Saintly Church: individual clergy may be corrupt or unpleasant, but the church as a whole is a force for good, providing education and charity to the poor of the city.
The entire plot of each Odd Thomas book is the titular character foiling one of these. The first book? Satanists. The second? A crazy woman that has studied evil religions all over the world. The third? A guy that claims he can create life. The latest has apparently radical Islamics that planned to assassinate important government officials before blowing up several American cities with nukes.
David Weber's WarGod series has a pantheon of evil gods.
The Fire Temples in the volcanic land of Krath. Despite being a theocracy, none of the natives are willing to talk about their religion at all, and laws in the cities are quite prohibitive. They trade for slaves to act as "Servants of the God." These "servants" are sacrifices which are roasted and then served in chunks to the people of the cities. There's a reason the local pirates fight to the last man to avoid capture.
Averted with Armagh's Satanists. They're the victors of a nasty schism on an all-Catholic planet, where they simply decided not to fight being called minions of Satan by a vicious inquisition. Their doctrine holds that God is being held prisoner by the angels, and the rebel armies of Satan will liberate him on judgement day. Other than the bizarre terminology in worshiping His Wickedness, they're good people.
One of the most Badass Evil Church of Evils around would be the pantheon in Lord of Light, by Roger Zelazny. Not only did the evil gods have all the psionic superpowers of the Hindu gods, but if you wanted to be reincarnated in a younger body, then you had your mind scanned for disloyal thoughts: have some and BAMF! you're reborn in the body of a smelly ape.
The Druids of his own isle of Erin had strange dark rites of worship, but nothing like this. Dark trees shut in this grim scene, lit by a single torch. Through the branches moaned an eerie night-wind. Cormac was alone among men of a strange race and he had just seen the heart of a man ripped from his still pulsing body.
In Ape and Essence by Aldous Huxley, the post-apocalyptic Los Angeles is ruled by Belial worshipers. The devastation wrought by World War III made them lose belief in any deity but a malevolent one.
Played with in the Books Of Swords: some of the gods, most notably Mars and Vulcan, are clearly malevolent, even evil. Interestingly, however, even though religion clearly exists in this world, we see very little organized religion. The only exceptions are the White Temple, which worships Ardneh, the Blue Temple, which ostensibly worships Tyche, and the Red Temple, which ostensibly worships Bacchus and Venus. Practically speaking, however, the White Temple is really a chain of hospitals, the Blue Temple is really just a bank, and the Red Temple is a chain of casinos and attached brothels. All three continue to function long after the gods they worship are dead. There is a reference in one book to a wizard bringing magical sacrifices or offerings to Mars to win his intercession in a battle, but that's about it for the explicit worship of any of the gods.
In the Empire of the East trilogy, however, it is made clear that Orcus, the demon prince, did compel his followers to worship him, so the titular empire was itself a Religion of Evil, at least until Orcus' top lieutenants overthrew him in a coup.
Taylor Anderson's Destroyermen series has the Holy Dominion, an empire located on an alternate America founded by Spanish conquistadors from our world mixed with a Mayincatec tribe (also from our world). After many centuries, the Dominion's version of Catholicism has adopted several Mayincatec facets, including the cult of pain and blood sacrifices. Basically, they believe that Jesus endured pain and spilled blood for our sins, which must be repaid in kind. Thus, in order to be close to God, so must everyone else. Only painful death in God's name can bring one closer to Him. Naturally, they believe that everyone should worship this religion, and the Dominion's blood cardinals spread the message. The author constantly insists that this is most definitely an evil perversion of Real Life Catholicism, especially since one of the good characters is a Catholic nun from our world. Another character, an Omnidisciplinary Scientist, is also a Catholic, although there's constant friction between him and Sister Audrey, who doesn't believe that an evolutionist has any right to call himself Catholic.
In The Dresden Files, it's mentioned that the Red Court encouraged the Aztec religion's notorious propensity for Human Sacrifice to keep themselves as well-fed as possible, with their highest nobility taking the place of gods.
The underground demon worshiping cult from Of Fear and Faith that tried to sacrifice Elin was definitely this. Elin sees them sacrifice another woman and summon a demon inside her that rips its way out of her chest and then devours her body. All in all, not a nice bunch of folk.
In the Discworld, the Guild premises belonging to the Fools and Clowns were formerly the Monastery Of The Brotherhood of Infernal Zoth, the Undying Renderer. This religion ended abruptly one night when the buildings were wiped out by a localised earthquake, lightning, and mysterious fire.
Grubindy's streetspeakers of Fate in Fates Road kidnap children from their parents if their parents don't follow their teachings. They also worship Fate, which is known in Grubindy for a) trapping them underground and b) causing their cavern to cave in and crush half the town.
On True Blood, Maryann Forrester is a Maenad who mind-controls humans into worshiping Dionysus (Greek god of madness and ecstasy) through wild orgies and ritual sacrifice.
The Sanguinista Movement, a group of Vampires who follow the Vampire Bible and fanatically worship Lilith. Their belief is that humans are food and nothing else, the complete opposite to the mainstreaming Vampires.
Stargate SG-1 has the religion of Origin, which while not evil in an of itself, is run by Alterans that have managed to Ascend. Its Priors also twist the meaning of the religion's holy book to justify mass murder and conquering other people whether they want to convert to Origin or not.
The Harbingers who worship the First Evil in Buffy the Vampire Slayer, and every cult of the series. At least one Wolfram & Hart security guard/soldier told Angel that he believed, specifically, in the cause of evil, by name. Angel using the guy's own shotgun to blow his head off did make others at least briefly re-evaluate their positions, however.
An episode of Criminal Minds featured a cannibalistic villain who worshipped Satan (or some kind of demonic entity). During the raid on his house, they find a hidden, demonic shrine, the walls of which were covered in Francisco Goya's Black Paintings and creepy symbols and Madness Mantras written in blood.
It's notable that the profilers state "He doesn't kill because he believes in Satan, he believes in Satan because he kills."
Lexx is built on this the first two seasons, with this "religion" being enforced on at least 20,000 worlds, in the goddamn Light Universe, even going to the almost absurd levels of the quote above, the only difference being that that is a parody, where this is a black comedy. Turns out it's just the ploy of an insect civilization to destroy humanity from the inside. And it worked.
The Disciples of Saxon in The End of Time worship The Master as a living god and ultimately bring about his resurrection.
In Robin of Sherwood there are three-and-a-half of these. First, Lilith and her unnamed companion who serve Simon de Belleme in the worship of Aziel (Simon's in it for the power, and his other servants are bewitched slaves, but Lilith seems to be a devotee). Second, the Lucifer-worshipping nuns in "The Swords of Wayland". The odd one out is "Cromm Cruac": the villagers indulge in human sacrifice, but they get an idyllic life in exchange - it's self-interest, not For the Evulz. Gulnar, that time round, gets involved for the sake of revenge - again, a normal human motive. But in Gulnar's next appearance, "The Time of the Wolf", he's running a murderous apocalyptic cult that really does seem to be just For the Evulz.
The Cylons themselves occasionally veer into this with their religious fanaticism.
'The Following'' features "Carrollism," a cult founded by Serial Killer Joseph Carroll. Loosely based on the works of Edgar Allan Poe and general Gothic sentiments, it depicts murder as an artistic/creative act, and encourages practitioners to "find their voice," by coming up with their own signature murder style.
Atlantis: The Cult of Dionysus is this. Your average kidnapping, brainwashing, and human sacrificing cult with a penchant for Ominous Greek Chanting and owning a pack of trolls to set upon strangers.
The pagans in Reign although there's reason to believe it wasn't always like that. Once it worshipped nature now it's centered around an unknown and presumably demonic entity in a cave somewhere in the Blood Forest.
In the Masters Of Horror episode "The Screwfly Solution", the Sons of Adam are a psychotic fundamentalist Christian cult who claim they are dedicated to 'freeing' the world of all female presence and restore it as it was in the Garden of Eden before God created Eve.
Other major examples in the standard Third Edition D&D cosmology are the churches of Hextor, Nerull, Erythnul, Gruumsh, Maglubiyet, Tiamat, and any of the Demon Lords and Archdevils.
Hextor's church falls between this and Path of Inspiration. Where his clerics have power, they openly preach his creed of militarism, conquest and tyranny. In other places, they claim he's a god of fitness, discipline and strength.
Subverted in Eberron with the death/ancestor worship of the Aerenal Elves. Plenty of trappings that would be red flags in most settings (skull motifs, efforts of worshippers to look more dead, fallen troops remaining on duty) but they are as benevolent (if somewhat less universalist) as the Silver Flame (not counting the Knight Templars in the organization).
... And then played straight with the Blood of Vol. Uncharacteristically, the setting information tries to be fair to those guys, despite their obvious black-hat practices like human sacrifice, necromancy, and immortality experiments. The cult's guiding moral philosophy is explained at great length and often from a sympathetic perspective; the faithful truly believe that this life is a torment and undeath is an acceptable escape. It's almost too bad the cult was founded by a diabolically evil lich who is using it for her own nefarious ends. Turns out she co-opted a pre-existing faith, and repackaged it into something more organized, and under her control.
Interestingly, becoming a thinking undead is actually discouraged for the majority of the the faithful. This is because the Bloods' faith places a lot of importance on the potential divinity inside a mortal — the undead are seen as having given up their chance for true divinity in order to protect and guide the faithful (also, most Blood-followers don't actually regard life as a torment, it's the death part that is seen as pointlessly cruel [which makes sense considering what the known afterlife looks like in Eberron]).
The Dark Six are a classical pantheon of evil deities cast away from the main pantheon... and The Traveller. It's possibly subverted, as the Dark Six could begods of nature that were removed from the Sovereign Host not because they were evil, but because the Sovereign Host was becoming the religion of civilization. Which, together with Eberron's Absent Deities-situation, leads to their evil possibly being a result of the Dark Six-worshippers gradually, over the ages, beginning to believe the propaganda spread about their deities.
A better example of being played straight is the typical Khyber Cult. They don't worship Khyber, just whatever they run across.
Evil faiths in Ravenloft tend more toward the Path of Inspiration model, but there are a number of minor cults dedicated to various Religions Of Evil imported from other gameworlds via the Mists. Of faiths native to the Land of Mists, that of the Wolf God (divine patron of werewolves) is the best example of an openly-bloodthirsty, destructive faith that looks to lay waste to civilization.
The worst example of this trope in Ravenloft (possibly the worst example period) is the cult of the god Zhakata in the realm of G'Henna. Zhakata is not even a real god. He only exists in the deluded mind of the realm's mad Lord, Yagno Petrovna. Followers of this religion believe that if they starve themselves, it will eventually bring about the coming of Zhakata in his role as the Provider, bringing endless prosperity and bounty. This had turned G'Henna into a poverty-stricken wasteland, with its citizens waiting for a pretend savior that will never come.
Planescape's Factions throw a wrench into this. There are quite a few of them who follow a belief system most of us wouldn't hesitate to describe as "evil" if we came across it in reality, but Planescape does its best to portray them from a detached, non-judgmental standpoint, often with inspiration from real life. The ridiculously callous and selfish Fated justify themselves based on, essentially, the writings of Nietzsche and Ayn Rand. The Dustmen, who have all of the trappings of a Religion of Evil (decorating things with skulls and wearing black robes and using zombies and such) have a belief system that is Buddhism with the added twist that they believe they've already lived their life and are now trapped in an existence of rebirth into a state of "False Death" until their souls learn to let go of suffering and attachment. The Doomguard has a philosophy centered around the inevitability of entropy, which can range from Omnicidal Maniac behavior to a simple acceptance that nothing can be accomplished without destroying something else.
Ironically, the Harmonium - a group known for being Lawful Stupid at best and a bunch of Knight Templars at worst - admire St. Cuthbert the God of Justice, regarding him as their patron god. Whether St. Cuthbert approves - or cares - about this isn't know, although their are many clerics of his clergy among the Harmonium.
Interestingly enough, in the default D&D cosmology evil people generally still go to the so-called Lower Planes (assorted afterlife dimensions, although only one group is considered "Hell" proper) after death to be tortured by the local denizens. If one is sharp and tough, however, he/she can survive and become more powerful. Most denizens of the Lower Planes don't envision themselves in any other place — with the notable exception of the prison plane of Carceri, where no-one wants to end up, and everyone who's already there wants out.
Fiendish Codex II notes that most evil people are egotistical— they don't look at lemures or dretches (bottom-of-the-barrel devils and demons, respectively) and think that will be their afterlife. They think they'll shoot to the top of the infernal hierarchies immediately. In addition, resurrection magic doesn't generally leave the revived with memories of where their soul ended up, so nobody has firsthand information as to what happens after you get killed.
Both subverted and played straight in the new fourth edition of D&D. Some evil gods actually have large (rational) followings that aren't typically seen as "evil". For example, non-evil worshippers of Bane (god of tyranny) might see him as the patron of "rightful and strong authority", and Grummsh (god of savagery) is worshipped by many as the "god of strength and conquest". A new deity, the Raven Queen, is worshipped as a personification of Death without the evil overtones (and is a mixture of Wee Jas and Morr from Warhammer). On the other hand, for those who like this trope straight, we still have Lolth, Vecna, Torog, etc., and also a lot of classic Demon Princes who are worshipped as gods by deranged cultists. Some of the more "evil" gods can be justified: for example, Torog's domains include jailers and torturers— necessary? Perhaps. Likeable? No, not at all.
The Raven Queen also has a good bit of My Species Doth Protest Too Much, as plenty of her worshippers are a bit overenthusiastic. (Shadar-kai especially tend to end up as random encounters.)
The one that takes the cake is probably Tharizdun, the god of omnicidal mania who has been imprisoned since the dawn of time by all the other deities (good, neutral, and evil) working together. He still has worshipers.
In the New World of Darkness game Vampire: The Requiem there's Belial's Brood, a covenant of vampires that are debauched and cruel even by vampire standards. And this is a Darker and EdgierCrapsack World where being a Friendly Neighborhood Vampire is nothing but a pipe dream. So trust me: "Debauched and cruel by vampire standards" means they aren't so much at the bottom of the barrel as outside of it holding up a liquor store. Their cults believe that the inner "Beast" that all vampires struggle with is actually a fragment of divinity caged by imperfect humanity; so, in an effort to "liberate" and master the Beast, they deliberately lead their followers in acts that drive the Karma Meter down faster than mercury in Antarctica.
The Brood are a Religion of Eviler to other vampires, while the Lancea Sanctum have set themselves up as a Religion of Evil to humanity, as they act as a dark mirror of Christianity (and other mortal faiths to an extent). Their message is "don't sin against God, or you will be cursed like us."
The Circle of the Crone, being vampiric Neo Paganism, somehow manages to pull this off, to the extent that learning dots in their Discipline causes a permanent reduction in the vampire's maximum Humanity score. Admittedly, this might be due to Early-Installment Weirdness.
Mythologies introduces the optional Mithraism heresy, which is unabashedly this. Simply gaining levels in a Mithraic cult requires acts of evil depending on initiation level; these range, in order, from utterly destroying an aspect of a target's life, to shattering a target's will, to driving a target over the Moral Event Horizon on-purpose, to engaging in a mass slaughter of human victims with nothing but one's natural weapons, to using treachery to indirectly kill another vampire, to purposefully committing Diablerie upon a more powerful vampire whilst they are being consumed by full sunlight.
Werewolf: The Apocalypse has the Black Spiral Dancers, a tribe of Garou who worship the Wyrm (the spirit of decay and destruction). The Black Spiral Dancers have their own pantheon, rites, and a sophisticated theology, as seen in Chronicles of the Black Labyrinth.
The first edition also features the Seventh Generation, a human cult that serves the Wyrm's Defiler aspect.
The game's most enduring example are the Baali, an ancient sect of infernalists considered vile even by kindred standards.
And then there's the Sabbat, who could be considered the predecessors of both the Lancea Sanctum and Belial's Brood. Many of them worship Caine, the first vampire, and believe that during Gehenna, he will rise up and save his childer from the Antediluvians that mean to devour them. Their membership consists of the most amoral vampires (and that's saying something), who often have to pick up alternate Paths of Enlightenment to avoid falling to their Beasts and practice twisted rituals that often involve human sacrifice (Their interpretation of the rite of baptism, for instance, requires immersion in a literal bath of blood). And the real kicker? They fucking hate infernalists.
Enter worship of any of the Gods of Chaos in Warhammer and Warhammer 40,000 and you hit Religion of Evil levels very quickly. Each Chaos God has their own angle: Slaanesh worshippers pursue emotional excesses, Khorne worshippers seek only to spill blood in Khorne's name, Nurgle worshippers spread disease and despair, and Tzeentch worshippers are fuelled by ambition and seek to evolve and change. The downside is that there is a roughly 99% chance of getting possessed, sacrificed, burned by witch-hunters or simply turning into a screeching, frothing, mindless mutant abomination with four heads that vomit blue fire. Absolute power corrupts absolutely, but only Chaos gives you eyes inside your mouth.
There's also the other side of that coin; the Gods of Chaos are the gods of bloodshed, disease, sexual perversion, and mutation. They're also the gods of Honor, Fortitude, Love, and Hope. As you can probably imagine, it's pretty damned hard to out-crapsack the Crapsack Universe of 40K.
That said, as mentioned by Ciaphas Cain (in Sandy Mitchell's The Traitor's Hand), many Chaos cultists start out presuming the organizations they're joining are relatively innocuous like crime networks, interesting occult or deviant pleasure groups, or even social reform movements; they often only grasp the true nature of what they've joined when they're far too corrupted to even care anymore. The cult in that book mostly consisted of the bored, and very few of them had any idea what they were getting themselves into.
At least in Warhammer, many of the cults initially present themselves as organizations for social and political change, something that the Empire could rather use.
Because of the way Chaos works (having an open mind will leave you defenseless against demons), Chaos often infiltrates and takes over actual rebellions and social reform movements.
Chaos Cultists tend to view the mutations and horrible deformities as blessings. There does exist for every follower of chaos a very small chance of getting immortality by being promoted to a demonic demi-god. Most of them die or go insane way before ever getting close, but as with the D&D example above, most don't look at the likelihood of death or spawnhood and think "hey, that could be me"; they're convinced that they will be among those lucky enough to ascend to daemonhood.
The Word Bearers Legion take this trope to its logical conclusion. In fact, Lorgar devoting himself to his official Religion of Evil is responsible for the 40kverse being the Crapsack Universe it currently is; it was Lorgar's minions who secretly manipulated events so that the Horus Heresy was inevitable — it was even a Word Bearer Chaplain who conducted the Chaos ritual that ultimately saw Horus corrupted and sworn to Chaos.
The best part? The other religions aren't that much better. At least Chaos is sort of honest about what it does, and doesn't pretend to be good.
In fact, the Imperial Cult worshipped by the Imperium? Created by Lorgar, primarch of the Word Bearers, as his first try at organized religion; when the Emperor smacked him upside the head with the fact he did not want any religions in his Imperium, even — or especially — if those religions deified him as a God Emperor, Lorgar went running into the arms of the Chaos Gods.
The most interesting legacy of Lorgar may be the Chaplains, the unique sect of Warrior Priests amongst the Space Marines. Their official purpose is to help keep Space Marines from being corrupted; however, not only were they created by the very first Legion to be corrupted (none of the other Traitor Legions have fallen Chaplains amongst their ranks because Chaplains were strictly found amongst the religiously-zealous Word Bearers), they were actually instrumental in corrupting the other Traitor Legions. Nobody remembers this fact now...
A small cult in the setting of the REIGN RPG believes in heaven for the good, hell for the evil and reincarnation for everyone else. They also believe that if you are killed by the archery-based martial arts style they invented, you get "promoted" in the afterlife. Thus, if you kill an evil person, they come back and get another chance, while killing anyone else gets them into heaven before they can do anything bad! They killed a lot of people.
Such cults fit in well in the world where KULT is set in.
Exalted features cults worshipping the Yozis, the titanic Demon Princes who created the world but were overthrown by the gods. Yozis are insane and evil, horribly twisted by their rage against the gods, and they want to corrupt Creation into a horribly painful hellhole. If they ever escaped their prison-world and remade the world the way they now want, it would be a Fate Worse than Death for all humanity. Indeed, before they were overthrown, humans were one of the most miserable species in existence. Nonetheless, some humans worship them and work to free them.
The Yozi generally don't play up the "We're going to wreck the world" angle. Most cult leaders tell their followers that the Yozi are going to fix the sorry state of Creation...Or just say they NEED to wreck the world first in order to fix it. When you spend your entire life farming and toiling just for another day of life while the Gods try to extort worship out of you, and the Exalted (Currently Dragon Blooded, formerly Solars) live like kings...A total change to the world where you don't toil just to keep breathing seems pretty damn appealing.
And, from a certain perspective, The Cult of the Illuminated.
The Dragon expansion features cultists who worship the various dragon lords, doing their bidding in the world by attacking the player characters. These cultists invariably receive a combat bonus when the particular dragon lord they worship is the dragon king, sometimes doubling their effectiveness in battle or psychic combat.
The Dragon Priestess player character is implied to have a leadership position in the aforementioned dragon cults, having the abilities to automatically take cultists as followers, and to make sacrifices to dragons they encounter in exchange for random benefits.
Other types of cultist enemies are thematically linked to various "boss" characters, such as the Ice Queen, the Eagle King, and the Dungeon Lord.
Breath of Fire II was one of the first games to effectively pull this off. The Church of Evrai is seen as good, but as it turns out, the high ranking officials are really monsters, Evan is actually a demon that worshipped the dark god Myria...aka Tyr.
Except possibly Mantorok, who seems to have posed as a relatively benevolent god for the locals.
The Cult of the Damned in Warcraft. Originally they were often seduced by promises of eternal life, social grievances, and disillusionment with the Holy Light. In the modern era this is exacerbated by the widespread trauma and suffering experienced by humanity. Members are given a potion after their initiation that removes their ability to object and makes them unquestioning fanatics.
And the followers of the demons of the Burning Legion from the same series.
Also the Twilight Hammer, who worship the cosmic horroresque Old Gods. The entire goal of their religion is to bring about the end of the world.
Final Fantasy VI: Okay, sure, the Cult of Kefka is composed of people who have had their homes and possessions reduced to dust, friends and families slaughteredjudged and world ruined by Kefka, and worshipping him is the only way you're allowed to keep your miserable life, so they're more The Church of Woobies than anything else, but really now; worshipping him (and being willing to go as far as to fight for him in order to escape his judgement) really isn't going to make it better.
The Order in Strife. They worship a malignant god, which involves blood sacrifices, mass murder, human processing and overall plenty of evil, demented stuff. Said god is actually an Eldritch Abomination from outer space which operates by draining worlds of all their life-force.
In Chrono Trigger, when the Fiends worship Magus in 1000 AD. The sole thing they desire is the death of all humanity. This is coupled with Ominous Latin Chanting from Fiends as they march around a statue of Magus.
The Lopto Sect of Fire Emblem Jugdral is a cult devoted to the dark dragon Loptous (A alternate spelling of Loki, Norse God of Evil), and a once-former Empire founded by a ex-Bishop named Galle who became disenchanted with the divine dragons and made a deal with Loptuous for power to conquer. They are rather keen on miss kidnapping of children to be used as human sacrifices to Loptous,and the tome that bears his name and was the source of Galle's power possesses the Final Boss, Prince Yurius, designed to be the dragon's vessel on Earth. All-in-all, a rather unpleaseant group.
In Mass Effect, the Geth religion comes down to the total extinction of all organic life in the galaxy. Unfortunately, the gods they worship — the Reapers — are quite real... And in an expansion of the trope The reaper Sovereign sees them as so inferior that it is insulted by the Geth's worship of it.
The second game reveals that the vast majority of the Geth prefer a "live and let live" approach, the ones that followed Saren being seen as heretics.
An indoctrinated Hanar combines this with Insane Troll Logic to come to the conclusion that since Hanar worship Protheans, and Protheans were turned into Collectors who now serve the Reapers, Hanar must worship the Reapers and aid them in their goal of total destruction.
In The Elder Scrolls IV: Oblivion, there's the Mythic Dawn, a cult lead by the Big Bad, who, among other things, require you to offer 'red drink' to Mehrunes Dagon (the god they worship) in order to join. There's also the Dark Brotherhood, who are a combination of a Religion of Evil, a Murder, Inc., and Knight Templars.
It's subverted with most Daedric cults who are pretty friendly while the God they worship has a different moral system instead of being evil.
However, there are some Obviously Evil Daedric cults such as those of Molag Bal, the King of Rape, worship of whom generally involves kidnapping and torture. His rival Boethiah, on the other hand, prefers to be worshiped through nice things like betrayal. When not functioning as a Big Bad, the cults of Mehrunes Dagon tend to fall into this category as well.
In Romancing SaGa the three Dark Gods each have followers, however the religion supporting the Big Bad is the Religion of Evil One of the Priests of the Temple of Elore in Melvir is actually the leader of said evil religion, even going so far as to summon monsters into the town and said summoning ritual was directly under the Temple of Elore itself, even more disturbing is the sacrifices and strange deaths at the start of the game
The Chzo Mythos series of games concerns The Order of Blessed Agonies, a cult whose members worship the eponymous Chzo. The cult has prayer books, religious texts, a symbol, a founder, and a fairly clear objective. Chzo is referred to as a "pain elemental", and appropriately, all of the order are masochists.
You might also call them a "religion of idiots", because they hadn't got a clue that their objective was completely wrong. They were also being fooled by their "god". Chzo just wanted a new prince, and the order were just a whole bunch of Unwitting Pawns.
At first glance, the Order of the Harvest Moon from the adventure game Harvester seems like a slightly skewed Brotherhood of Funny Hats. Once you've passed the final step of their initiation ritual, which involves navigating an Evil Tower of Ominousness, killing horrible monsters, and having sinister and nihilistic conversations with really nasty people, you've discovered their true colors... and their true intentions.
Lampshade Hanging in Star Control II: the Ilwrath are theocratic self-declared worshippers of Evil and Death, but if the player confronts them over this ("If your actions are judged by your society as correct, aren't you, in fact, good?") they tie themselves into a logical knot before deciding to attack the player for being annoying.
A bit of a subversion in Dead Space, as the Church of Unitology seems like this as you go through the game, what with them wanting bring the Marker back to Earth and turn everyone into Necromorphs, but you'll find somewhere in the middle of play that there are Unitologists that didn't think turning into Necromorphs was all that of a great idea, but Mercer killed them. Played straight in that this really seems to be how the Church and most Unitologists are.
The promise of a coming of angels that will touch the bodies of the dead to transform them into heavenly beings that are reborn in paradise doesn't sound so bad by itself. It's only when you realize that these "angels" are giant alien bats that will jam an arm-long spine into your skull and transform you into space zombies, which neither bother to make a difference between belivers and non-believers, wait for your natural death, or make your death any less painfull than being torn to pieces, that one starts to doubt that path to paradise is really such a good idea.
What really makes it ironic though, is that its "prophet" didn't start it, and in fact was opposed to it. He merely found the Black Marker and was immune to its effects, and people that were affected by it started worshiping him as a prophet. He began trying to blow the whistle on a government plot to use the Marker as a weapon. The two military officials that were in charge of the project kidnapped him and killed him with a Necromorph to martyr him in order to strengthen Unitology, which would most likely throw suspicion away from the government and quell social upheaval that Altman was causing.
By Dead Space 3, a radical Unitology group, the Circle, managed to catastrophically destabilize EarthGov. While EarthGov was corrupt and behind much of the evil in the franchise, the Circle's troops gun down civilians, deliberately cause Necromorph Outbreaks, and consistently cause trouble for our heroes. An in-game log shows that most of them were not aware of the Necromorphs at first, and that their leader rationalizes them not as a natural endpoint of their beliefs, but rather as a plague brought about by nonbelievers tampering in God's Domain.
Interestingly, the first game and its associated Expanded Universe have a few examples of "good" Unitologists, people who are not fanatically devoted to the religion and fight back against the Necromorphs because they realize this cannot be the salvation they were promised. Later games don't feature such individuals, perhaps explained as an effect of the Markers' growing influence.
The Nhuvisarum in the Summoner games. They enslave nations and purposely garner a 0% Approval Rating because their magical powers are fueled by human suffering.
Every dungeon in Exile/Avernum will have an evil temple with skulls, bloodstains, and/or traps that unleash demons in it somewhere.
Played straight and parodied in the Fable games with the Chapel of Skorm and the Temple of Shadows. The latter example involves a rather amusing parody. "On Wednesdays we drain the blood of virgin chickens. On Thursdays we anoint ourselves with said blood. Friday is poker night, of course"; and "If there's one downside to being an evil cultist, it's that we must take our tea without milk" indeed.
Dragon Quest V features the Order of Zugzwang, a cult that worships the Big Bad. Interestingly, it counts both monsters and humans amongst its ranks.
The Multitude in Incursion is a, well, multitude of demon ghosts who each want you to go kill and slaughter and rape and be generally depraved in that particular ghost's name, and are willing to bribe you with power for it. Zurvash is slightly more philosophical about it, making no bones about the fact that he has no regard for civilization, ethics, or foresight, and glorying in brutality and domination.
EVE Online:Religion of Amarr Empire approve of slavery, conquest and racial superiority. The Blood Raiders. Since their "religion" invovles vivisection with no anesthesia.... And to a lesser extent Sansa's Nation. They believe in turning people in to "true humans" by destroying the personality yet keeping intelligence and creativity intact.
The Order of the One True Way in Suikoden Tierkreis has elements those can qualify as Nightmare Fuels. For example, early in the game, none of its followers react, let alone tried to run at all, when lightning struck among the crowd and fried one of them to death. All because their leader has predicted the lightning strike.
The Thurists are very much this. Every member indulges in an Evil Laugh, even in the battle quips (ordinary units will have the laugh replace their normal grunts and cries), their "god" is World Eater Thuris, the sickest of the lot (mentally, that is - Raksha's a Magnificent Bastard and Feinne is simply mindless), and they regularly purge other religions, especially the followers of Apis, who many of their members were converted from. Danette is a survivor of one of these purges, which razed the town of Pulkina ten years ago. And if that wasn't bad enough, they have a reputation for indulging in the spread of Scarlet Iago - this may have killed Shauna/Shari and Trish's parents 15 years ago during a vaccine panic, and if it weren't for a charitable donation by Cristophe and Levin, it could have claimed the Raide survivors under Nereid care as well. Kanan and her lot all fail to comprehend why evil religions are doomed to swift destruction - namely, that they are mere pawns for the "gods" they worship.
Church of the Key in Alundra 2. They promise you happiness, they turn you into a soulless slave.
Legacy of Kain has a few examples, but only one truly fits. While the ancient vampires worshipped the soul devouring Elder God, they believed his wheel of fate was good and divine. The vampires of Kain's empire deified him after they conquered humanity, he was their creator. The priests of Avernus Cathedral(the offical religion of which is never explored in detail), however, are members of a secret cult worshipping Hash'ak'gik, a group of hylden possessing the body of a horribly mutated vampire from the future. They preformed ritual human sacrifice to their 'god', cutting the throats of their first born and throwing them into a massive pit.
Soul Reaver also briefly features a run-in with human vampire worshippers, who seemingly kidnap humans and feed them to their masters, presumably in the hope that they will one day be converted into vampires themselves. Either way, pretty evil.
One of the Dummied Out bits from Soul Reaver before it was gutted due to time constraits would have involved Raziel killing a human priestess in the last human settlement who was using mind control to send humans into a vampire den.
In Rise Of The Kasai the titular Kasai is an evil cult who's only purpose is to gather up six marks that are branded on to people's flesh, and use them to cast a spell that will allow their God of Evil to enter the world and wreak havok. Said marks are often found on children, who they will kill and skin to use the magic.
Dungeon Crawl features several Religions of Evil, from Beogh, god of the orcs, Kikubaaqudgha, an evil demon-god of necromantic magic, Makhleb, god of chaos and violence, to Yredelemnul, an evil god of death. Xom, god of true chaos, is an interesting case, in that he isn't looking for followers quite so much as new playthings.
In Metroid Prime 3, the corrupted Space Pirates certainly seem to regard Dark Samus like Jesus, referring to themselves as disciples.
The Order of the Silent Hill series worship a God that has promised to bring about the salvation of humanity and paradise on Earth. Given that their plans for helping God bring this about invariably involve physical and psychological torture and murder (especially of children and childhood best friends) and that all of the Order members (with the possible exception of Claudia, who can be seen as just a Well-Intentioned Extremist) encountered in the series are amoral, vicious sadists, and/or outright insane, one can be forgiven for wondering what exactly their paradise would entail.
Interestingly, their God is frequently depicted as female, and many of the church's evangelists are women. The menfolk, such as Kaufmann or Vincent, are more interested in earthly matters and tend to mutiny.
It's telling that the main antagonist of Silent Hill 3, the Order priestess Claudia Wolf, who orchestrates a brutal murder and has the protagonist put through Hell, would apparently be considered one of the Order's more liberal theologians.
Something else that makes the Order look like pure evil rather than just a Scary Amoral Religion is that their God thrives on suffering and hatred, although Claudia reasons that a truly compassionate God must first experience the horrors of the world through a human host.
Valkyria Chronicles II has Yggdism, a cult that worships the setting's resident omnicidalEldritch Abominations as gods and preaches that brutal oppression and mass slaughter of Darcsens should be a way of life. While not all that prevalent in the original, one of the main villains from the sequel is a devout follower of Yggdism.
Halo has an interesting example. While the religion of the Covenant didn't start out this way, thanks to the Prophets misinterpreting the sayings of the Oracle in their holy city ( actually a fragment of Mendicant Bias, a Forerunner AI who went nuts and joined the Flood for a while) and their own corruption and thirst for power, it soon turns the Covenant into an alliance of omnicidal maniacs. Granted, they didn't think they were going to be killed as well.
At the same time, their genocidal war against humanity was started by three Prophets who found out that humans are the true Reclaimers and not the Prophets. In order to keep the Covenant from collapsing and eliminating all evidence, they have declared humans evil and ordered their destruction. It was also a political move to put the three in power, and they become the Hierarchs we know and love.
In Mortal Kombat, the Brotherhood of Shadow follows the fallen Elder God Shinnok.
The White Mantle of Guild Wars: Prophecies starts out seeming like a bunch of pretty cool guys. They're a functional theocracy, wear nice outfits, welcome you into Kryta after your homeland gets burnt to a crisp, and induct you into their order. The only real problem is that the gods they worship are actually a race of amoral illusion-weavers who are currently harvesting the souls of specific individuals to power the seal on an entrance to the realm of a very real and very evil god. And they're not doing this because it's necessary for the greater good; they just want to make sure they themselves stay alive.
After the destruction of most of the Mursaat race by the opening of the entrance they'd kept sealed until some nosy heroes showed up, the White Mantle hired former bandits, thugs, and other detritus of society, named them Peacekeepers, and sent them out to kill anyone who didn't agree with the Mantle.
The Illians worship the winter god Mulcarn. Their goal is to turn everything to ice again, so they can prosper. Of course, no other civilization can survive on ice. After the death of their god, their task is to turn their leader into a new winter god.
The Path of Dark in the old Might and Magic verse is heavily implied to be either this or a Philosophy of Evil - it has adherents that self-identify as Villains, some members of it call you a do-gooder, or criticise your lack of cruelty if you happen to align with the Path of Light...
In addition to various examples from the usual suspects among the Forgotten Realms gods, Neverwinter Nights 2 has people who worship the King of Shadows (the Pure MagicBig Bad). The second expansion adds the cult of Zehir, worshipers of the yuan-ti god of poison who have set themselves up as the rival to a different Religion of Evil, those who worship Sseth, the yuan-ti's normal patron deity.
The first game, meanwhile, had the "People of the Eye", who were working to ressurect the Creator Race through such means as attempting genocide against an entire city through a Mystical Plague.
Runescape originally had only three gods, Saradomin, Zamorak, and Guthix, who were good, evil, and balance. Zamorak is considered more chaotic now, and has some followers who are decent and suggest that their reputation is Writtenbythe Winners. Still, the Zamorakians are arguably 99% evil, featuring all sorts of bloody murder, their members include vampires, werewolves, demons, and an Evil Chicken, and many of their rituals actually use the word evil.
The Legend of Heroes IV: A Tear of Vermillion has a rather complex complex backstory leading to the Religion of Evil. The leader, Bellias, was originally the next Supreme Priest of the Bardus Church before his epiphany in the Island of Kanaphia / Truth Isle, in which he turned to Octum as his god. The great irony of this Religion of Evil is that its base is the same city as the Bardus Cathedral and the portal to Octum (and base of operations) is right under the Cathedral. And just to demonstrate how evil this religion is, Bellias' second-in-command, Borgeid, is responsible for corrupting the Great Spirits and corrupting the lands. And they resort to terrorism in spreading their beliefs. Yeah, it's that evil.
There's a few of them in the Might and Magic series. In the first few games, there were a lot of monsters that suggested evil cults, but with no clues to any actual cults. Starting in VI, they had the Cult of Baa, which worshipped the Kreegans, devil-like creatures that were the antagonists. These cultists also appeared in VII.
In VIII, there were the Necromancers, but they were only enemies if you chose the Light Path. If you chose the Dark Path, they were important allies.
In IX they had some fun with this, and it might have been okay if the game as a whole wasn't so poorly done. The "evil cult" here was the Cult of the Great Honk. These guys weren't related to the main plot, and were Ineffectual Sympathetic Villains at best. (In fact, one of the quests involving them required you to convince a group of them to leave a city without killing them. (The Great Honk wasn't even a real god, it was a giant goose that the actual gods kept as a pet.
The Dimension of Pain demons in Sluggy Freelance worship the Demon King, who thinks the highest compliment he can give something is "How evil".
The same goes for the cult of K'Z'K started up by Chilus.
The doom cultists in 8-Bit Theater. And whatever Black Mage is worshipping that week. Though Black Mage admits he's only worshipping them for the express purpose of gaining their dark blessings until he is powerful enough to usurp them for his own ends. This comes back to bite him later on, when Chaos rises and he tries to get on his team. Chaos then tells him that he's planning on destroying everything, Black Mage asks if that includes him. Chaos's response? "Biiiiiiiiiiiig time". Needless to say, Black Mage immediately sees the need to kill Chaos.
Vince's cult in Jack is a weird example in that the cultists are all already in Hell, and are turning to the cult to try and get them out. Vince has a lot of fun manipulating them.
"Now each of you prove yourselves! Come to the centre of the arena and curse God's name! Denounce His only son! For only those with the courage to face God's wrath are worthy to bask in His glory!" They don't even think twice. No wonder they're damned.
In Homestuck, the troll version of Juggalos are a legitimate religion on Alternia. They possess terrifying psychological powers known as "Chucklevoodoos" and use the blood of trolls that they slaughter as decorative paint. And the Mirthful Messiahs of the religion? They are actually the Big Bad and his Dragon.
Subverted in The Order of the Stick: Despite the deity being called "The Dark One" and despite his worshipers being almost universally evil creatures, it's not quite a Religion of Evil but rather a religion of people who are so downtrodden that they're willing to do anything to even slightly improve their lot in life, even to the point of tempting The End of the World as We Know It. More a religion of evil people than a Religion of Evil.
Redcloak's brother Right-Eye eventually renounced the Dark One and the Plan because he believed the religion had become a straight example of this, with the Dark One caring more about getting revenge than about improving the lives of goblins.
Subverted in the Whateley Universe with the Cult of Kellith, worshiping the daughter of Gothmog (and granddaughter of Shub-Niggurath)... who, for all that she's ostensibly a lust demoness, started out as a human, is currently attending Whateley Academy as a student, and is generally not a bad person. (Although there are signs that she's starting to actively distance herself from her previous human existence and morals, so this may change in the future — she is a Cosmic Horror in the making, after all.) Played straight, however, with the Tong of the Black Madonna, an apparent mystical cult opposing the Tao and making trouble for its current Handmaid a.k.a. Bladedancer, including a concerted attempt to attack her in her dreams via a spell powered by human mass sacrifice.
The Cult of Kellith probably isn't a subversion, but a real example. The head of the cult was actively trying to turn Carmilla evil until she killed him. Cultists have been seen to do things like sacrifice cute animals in her name. And there may be a hundred thousand of them out there doing god only knows what.
Ezekiel Rage from Jonny Quest: The Real Adventures had the Book of Rage, a supposed bible from which all apocalyptic prophecies must come to pass, and which he worships fervently. Subverted in the fact that the book was actually empty, save for a picture of his dead family.