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"BLOOD FOR THE BLOOD GOD! SKULLS FOR THE SKULL THRONE!"
"I am called a villain, but mine is a doctrine reflected in the unspoken truths behind all other philosophies. Recognize that truth, my child: Strife is the single element most essential to meaning and existence. Without conflict, without struggle, all of creation might as well be so many unmoving, unfeeling stones. Without me to hate, who would have cause to call themselves holy or just?"
Jashin of Naruto means "Evil God". He/she/it is mostly an excuse to allow his follower Hidan to practice his Religion of EvilBlood Magic (read: fight and kill people). The Ten-Tails also counts, despite being the progenitor of all that exists in the world.
Ruby-Eye Shabranigdo in Slayers, along with three other Dark Lords that exist in The Multiverse it features. Before the world as everyone knows it existed, Shabranigdo started a war against the Lords of Light. The war almost destroyed existence itself, but the Lords of Light prevailed and the world was created.
Dark Star Dugradigdu in Lost Universe is named after one of the above-mentioned Dark Lords, and is sometimes believed to be one and the same as him.
The Idea of Evil and the God Hand from Berserk. The former is interesting in that is was made from our need for there to be a God of Evil.
Zorc/Zork Necrophades (spelling varies on translation), the Eldritch Abomination from Yu-Gi-Oh!. He's the god of darkness and ultimately the main villain of the series, [who either controlled or was directly allied with some of the biggest villains earlier.
Darkseid, the Big Bad of Jack Kirby's New Gods, has been called the God of Evil by many people in the DCU. And in Final Crisis he comes close to being a full-blown Eldritch Abomination — his very existence is causing reality to die. Just to bring this point home, here's what Boss Dark Side says: "There was a war in Heaven. I won." He is the poster boy for Dystopia Justifies the Means- his goal is to turn the entire universe into an even more hellish version of Apokolips, a world of perpetual enslavement and burning fire pits where the only purpose you are told to have is to endlessly worship Darkseid.
The Elder Gods, powerful magical entities native to Earth, born billions of years ago shortly after the planet was created. While they did not originate as evil, they began cannibalizing each other once they found out that they could gain more power that way, eventually degenerating into extremely powerful demons. Only Gaea and Oshtur did not join their brethren; the latter fled to another dimension and eventually became one of the Vishanti, while the former birthed the entity Atum which proceeded to hunt down and eat the Elder Gods themselves. A handful escaped into other dimensions and Gaea and Atum set up numerous magical barriers to hamper their returning, while Atum degenerated himself into the evil creature the Demogorge due to the demons he consumed; he expelled this from himself and it became The Legions of Hell- demons such as Thog and Mephisto-, while he retired to live in the sun. The Elder Gods include:
Set, a multi-headed serpent who was the first to cannibalise, and the demon partly responsible for the death of the dinosaurs; he managed to bypass the barriers and started feasting on the life energies of the dinos, but after learning Gaea planned on making them extinct and starting over with mammals, used mind control to send them to kill all mammilian creatures. Gaea responded by waking up Atum and the battle between the two, apart from costing Set his original head (two more kept replacing them when Atum cut them off, until he had seven), ended up causing the extinction event itself and left Set so weakened he had to flee again. He later returned as a major recurring villain during Marvels take on Conan the Barbarian.
Chthon, the writer of the Darkhold, an evil magical tome responsible for, amongst other things, the creation of vampires; the god of Chaos, and more specifically chaos magic. He is the source of the power of the Scarlet Witch, with the long term plan of using her as a host, and is currently sealed away in Mount Wundagore (or at least, part of him is), which became the magical (and irradiated) source of clay for Fantastic Four villain the Puppet Master. He is the Evil Sorceror of the group.
Aside from the Elders, the regular gods are divided into pantheons, each of which has their own bad guys:
Loki, foster brother and Arch-Enemy of The Mighty Thor, is the God of Mischief and Evil in Marvel's Norse pantheon, a powerful magician and Manipulative Bastard extraordinaire who seeks to conquer Asgard and kill his heroic sibling. He is technically not a god, but a very small, god-sized Frost Giant, adopted by Odin as a babe as penance after Odin slew his father in battle. Loki bares no real grudge for that (in the comics, anyway), but instead grew resentful because he thought Thor was treated as the favourite, especially by other Asgardians. As boys, he often tricked Thor into getting into dangerous situations hoping he would die in the process, and as full grown gods got him banished from Asgard on several occasions. Accidentally caused the formation of The Avengers when he pitted Thor against the Incredible Hulk, and remains one of their most persistent and powerful adversaries, often by giving common thugs superpowers and sending them into battle against them, or using his magic to cause all sorts of trouble. He is the father of various evil creatures as well as Hela, Goddess of Death, who borders on an Everybody Hates Hades evil god herself.
Amatsu-Mikaboshi, for the Shinto pantheon, a demonic creature and a malevolent Death God (not their only death god, just their bad one) that serves as the God of Evil for those gods, and their most primal enemy. He/ she/ it is the Anthropomorphic Personification of the primordial darkness that once ruled the Earth after the defeat of the Elder Gods; prior to that, it once ruled the Marvel Universe and prior to that, it ruled the universe preceding that universe, and originally was the nothingness before existence. Over time it had to constantly descend as it dealt with the various new creatures and gods that peppered the planet It regained some of its old stature by slaughtering, eating and enslaving hundreds of gods and demons (notably Zeus and Nightmare) to become the Chaos King, where it (re)joined the cosmic pantheon alongside Eternity and Death, seeking to destroy the multiverse and return it all to primordial darkness. At full power it just falls short of being some kind of Anti-God, though it might be more accurate to say it is anti-Creation, preferring to exist in and rule over a chaotic nothingness.
Set, not the Elder God but the God of Evil for the Heliopolitan (ie. Egyptian) pantheon, based on the mythological Set but, in-universe, taking the name of the Elder God for its badass connotations. Like Set and Loki, he uses magic and soul-stealing to enhance his powers, to the point he now rivals Odin, arguably the most powerful god in any pantheon.
Everybody Hates Hades and, as Pluto, this Death God of the Greek pantheon serves as their resident bad guy, and enemy of Marvels' Hercules. In DC he is portrayed more faithfully as a more neutral figure. Aside from him there is Ares, God of War, though in recent years Ares has become an Ascended Extra and morphed into an Anti-Hero, albeit a very dark and violent one.
Eldritch Abominations like Shuma-Gorath and Dormammu are worshiped as gods in the dimensions they rule over, and numerous others, and wield nigh-omnipotent power arguably above every other god previously mentioned, at least at full strength and in their own worlds. For both Shuma-Gorrath and Dormammu, worship serves as a source of power, though for Shuma-Gorath, both his power and physical size is based off how much fear he can induce in the realm (in his native Chaos Dimension, he is larger than a galaxy), and commanding regular human sacrifices worked well enough. Both seek to conquer the Earth- Shuma-Gorath has actually already done this, twice (first during the age that Conan the Barbarian takes place in), in ancient times- and can be guaranteed to torture and kill everything on the planet/ universe if they ever succeed. Hell Lords like Mephisto wield godlike power within their dimensions, but are not treated as gods per say, or seemingly worshiped in any way except by the odd gullible mortal. In fact, the description of the Hell Lords listed on Satanic Archetype explains why NONE of them ever try to become the one true god of evil.
Some other Marvel supervillains are sometimes worshiped as gods themselves, most notably Apocalypse who was, as it happens, also mistaken for Set and various other gods throughout his immortal lifespan, and would use this influence to force civilizations to fight to the death until only the strongest survived. In the Legacy novel series Magneto was worshiped as God on Genosha, while Doctor Doom has successfully stolen godlike power numerous times from cosmic beings, though on such occasions he prefers to be actively worshiped as a King or Emperor rather than a deity. Selene, another X-Men villain, is a 17,000 year old vampiric mutant sorceress named after and sometimes worshiped as a goddess, the attainment of which is her ultimate goal. The Green Goblin once thought he was one of these, but he was actually just driven mad(der) by an Artifact of Doom.
The Big Bad of Fear Itself the Serpent is the Asgardian god of fear and Odin's brother Cul Borson.
Anathos from Les Légendaires; he was part of the Gods who created Alysia, but turned against them because he wanted to become a "Mastergod", destroyed the original Alysia, was sealed away by them after his defeat and came back in order to attempt The End of the World as We Know It. He even stated to have created this universe's version of Hell.
There's also Father Deer, the Hooviets' deity. He's basically said to be the embodiment of their 'Greater Good' philosophy...which is just a fancy way to say 'destroy freedom, stomp all non-deer races under an iron hoof, and Take Over the World'. However, there's question in universe as to whether or not he even exists.
Eugenesis mentions that the Quintessons have five gods, all of them as twisted as the Quints. Case in point, one of them is Unicron himself.
La Magra, the vampire Blood God from the Blade movies. The evil scheme in the first film is to complete a ritual to summon him into a vampire avatar so he can walk the earth. If he fights humanity, he will turn everyone into vampires by his mere presence. Deacon Frost summons La Magra into himself and becomes much more powerful, being made entirely of blood and showing insane regenerative powers. Blade kills Frost by loading him with anti-coagulents, but it's unknown if this killed La Magra too.
The Dark One from The Wheel of Time is one of the quintessential examples of this trope.
Lord Foul the Despiser. He was imprisoned with Time by his opposite, the Creator, for planting various sources of evil magic into the universe shortly following the creation, and he's been trying to get out ever since- and because he's super-intelligent and unrepentantly horrible, this generally means misery for everyone else unlucky enough to be stuck in the Land with him.
Ruin from Mistborn is something of a subversion- he's definitely a god and as far as humans are concerned he's pretty clearly evil, but he doesn't think of himself as such and isn't interested in either ruling people or corrupting them. He does use mortal pawns, but some of those include the heroes doing heroic things that happen to speed along the destruction of the world. He does have some unequivocally evil tendencies (namely a fondness for Evil Gloating), but these seem to have been picked up from mortals rather than the reverse.
There's a third of the Dragonlance pantheon called the Gods of Darkness, and they're, well, Exactly What It Says on the Tin. In something of a subversion, they're considered nasty but necessary parts of the cosmos, though every so often one of them (usually Takhisis or her consort Sargonnas) gets a bit too ambitious and needs to be put back in their place.
Kushiel's Avatar by Jacqueline Carey involves Angra Mainyu, taken from real life Zoroastrianism. (See the Real Life section below for more details.)
In Animorphs, Crayak is effectively this - at the very least, he plays The Social Darwinist on a cosmic scale, using shock troops like the Howlers to weed out weaker species.
David Eddings likes this one. In his Belgariad-verse, Big Bad Torak is one of these and losely follows the Satan model, as does the mentioned but never directly confronted King of Hell. In The Elenium, Azash is an elder evil that verges on Eldritch Abomination if he didn't manipulate human desires so often (though it's clear his understanding of humanity is imperfect, and largely based on the cultural group most of his worshippers come from), while Cyrgon is a straight example and Klael is the god of Gods of Evil. To round the roster out, The Redemption of Althalus has Daeva, a god of destruction who morphed into this when he promoted himself from "takes out the cosmic trash" to "Omnicidal Maniac".
The Crimson King from Stephen King's The Dark Tower series definitely qualifies. Before he got nerfed he was depicted as an invincible anti-God that had always existed, but was trapped on a balcony near the top of the Dark Tower. Even trapped his avatar "The Court of the Crimson King" was so powerful that almost no force in King's multiverse could stand against it. Supposedly if his physical body was free he would become utterly infallible, and so he planned to bring the Tower (and by proxy all of existence) crashing down. Later King retconned him into a senile half-god that was not nearly so imposing.
The Incarnations of Immortality series has this as one of its offices. However, the particular office holder in the series averts this; he's less interested in being a relentless evil and more interested in simply being a fair evil - that is, separating good and evil people through temptation so that they can go to Heaven or Hell rather than stuck in Purgatory.
In L. Jagi Lamplighter's Prospero Lost, Theo recounts how the Orbis Suleimani eradicated knowledge of the supernatural. This was to prevent people from trying to solve problems by appealing to supernatural beings, many of which demanded worship and some of which were evil. Success resulted in the Industrial Revolution.
Tash is described this way in The Last Battle, the final book of The Chronicles of Narnia, with Crystal Dragon Jesus Aslan outright stating that Tash is his antithesis. Interestingly, while Tash is very much a Card-Carrying Villain, his own religion is not a Religion of Evil, given that it apparently includes a number of sincere followers who believe Tash to be a just and righteous god. It gets better - Tash is so evil that you cannot commit a good act in his name - they automatically are assumed as your service to Aslan. The opposite is also true - if you do something terrible in the name of Aslan, you're really serving Tash.
Aslan: No service which is vile can be done to me, and none which is not vile can be done to him.
Gyphon from Mithgar is both the overarching Big Bad of the series and its primary God of Evil. Specifically, he represents tyranny, since he holds to the belief that the strong should use their powers to control the weak and in fact opposed giving mortals (and non-divine immortals) free will in the first place. Other, lesser Gods of Evil who follow him are namedropped occasionally, but none of them ever do anything directly and very little is ever revealed about them.
The "nuclear chaos" and "daemon sultan" at the center of the universe, the Outer God Azathoth of H. P. Lovecraft'sCthulhu Mythos is often at times interpreted as this. Judging by what would happen if Azathoth's literary inspiration, Mana-Yood-Sushaiawakens, it would likely seem that Azathoth has the power to end everything in the universe, including all the other eldritch horrors - but for now just chooses not to. Throughout Lovecraft 'cannon' Azathoth is frequently referred to as the "blind idiot god", not because it is in any way stupid, but because you would have to be a blind idiot to worship it.
The Crippled God Deconstructs this trope. He essentially fills this niche, but he's more like a god of suffering than evil- and while he certainly inflicts more than his fair share of suffering on the world, he also continually suffers himself. In the end, after a bunch of AbusivePrecursorKnight Templars hijack his power, he ends up becoming a more sympathetic deity who empathizes with those who suffer.
Bellurdan Skullcrusher, the Dying God, is the trope more or less played straight, corrupting people with his toxic blood into becoming his followers.
The High Fantasy metaseries consisting of The Scared Hunt, The Sun Sword and The House War has Allasakar, more commonly known as the Lord of Hell or the Lord of Night (or, to his demons, simply "the Lord"). The most belligerent (and powerful) member of the pantheon, back when the gods still lived in the mortal world he attempted to conquer it; when the other gods left, they only convinced Allasakar to leave as well by giving him Hell has a consolation prizenote And because he was wounded in battle with a mortal champion, something he'd previously considered impossible. Owing to how the cosmology is set up, all souls are reincarnated through many lives until they either attain an enlightened state or become irreversibly corrupt; Allasakar gets the latter. His goal now? Assume dominion of the mortal world to make it impossible for souls to develop as they need to, ensuring that all souls will eventually end up in his domain.
Charmed had The Source of All Evil (usually just referred to as The Source), who was the ruler of the underworld and the Bigger Bad for the first half of the series' run. It was later revealed that "The Source" was actually a primordial force that lived inside whatever demon was currently leader of the underworld at the time.
There is also Fenric, who according to the Doctor is evil from the dawn of time. The expanded universe retconned Fenrir as being another name for the Lovecraftian diety Hastur.
Mythology & Religion
Some more dualistic forms of Zoroastrianism claim Angra Mainyu (better known by the Persian name Ahriman) is the Evil Counterpart to Ahura Mazda. In other words, he is essentially the anti-God, or the god of evil.
Furthermore on that topic, Angra Mainyu isn't so much a name as a title alluding to his nature. Angra can be translated as "destructive" or "malign", while Mainyu can mean "spirit" but also "mind", "concept" or "thought". Which means, Angra Mainyu is theIdea of Evil!
Egyptian Mythology has Apep (or Apophis), the monstrous serpent that tried to eat Ra every night and wasn't so much worshipped as worshipped against, straddling the line between God of Evil and Eldritch Abomination.
Set is portrayed as this to a degree in later versions of Egyptian Mythology, though contrary to popular opinion he wasn't a snake god, and was actually a good guy in earlier myths; Set lost ground as the Horus cult gained it in later centuries, and became more identified with foreigners and the desert, even becoming identified sometimes with the chaos-serpent he had used to fight. By the time Petrarch recorded the most familiar-to-moderns versions of the Egyptian gods, he was bidding fair for this role.
In Shinto Ama-no-Kagaseo is the Anthropomorphic Personification of Amatsu-Mikaboshi, which is essentially The Dark Side. So it is the personification of Evil, or at least discord, destruction and chaos, and destructive impulses.
Medieval Maori religion gives us Whiro, who manages to some way or another cause practically every problem we have while locked in the underworld. He will eventually escape and destroy everything besides himself and the ashes.
A religion that has conquered another religion often turns the old gods into demons. This is the origin of the word "demonize". And now you know.
Dragonlance, being almost entirely about the Balance Between Good and Evil, divides all its gods into gods of good, gods of evil, and gods of neutrality. Obviously, each of them does have the own unique spheres of influence above and beyond their alignment.
In the Forgotten Realms there are multiple evil gods, and like in Dragonlance they are more about malign things than actually being the incarnation of evil. The closest the setting has to a God of Evil would be Shar, the evil goddess of darkness and loss. She is the dark twin of Selune, good goddess of light and hope: According to Forgotten Realms lore, the twin goddesses were the first beings to come into existence in the Realms and created everything within it: They represent the primal forces of light and darkness in its purest form. Shar is currently more powerful than her twin because Selune's role has been eclipsed by various other good deities, while Shar's has remained inviolate.
Another applicant for the role as official god of evil was Jergal, the Lord of the End of Everything, who was responsible for all strife that plagued mortals while alive, for killing them, and for the afterlife (or, specifically, the afterlife of everyone who weren't spoken for by another god).
And in the Eberron setting, you've got the Dark Six, a whole pantheon of these (though they can have positive effects, they do have plenty of non-evil worshippers in certain parts of the world, and it's likely they were originally the gods of the goblin empire before humans moved in). There are also the Rakshasa Rajahs, ancient demonic beings who once ruled the world, each corresponding to a different kind of evil. The other main dark powers in the setting (the Dreaming Dark and the Daelkyr) are, however, closer to Eldritch Abominations.
Izrador of the Midnight setting is fairly standard as dark gods go. The other gods tried to banish him from heaven, but did not expect the unforeseen side effect that it would not only send him to the world of mortals, but also severed all connection between heaven and earth. Think of Sauron if he got his ring back, only worse.
The Chaos Gods of Warhammer and Warhammer 40,000 straddle the line between this trope and The Heartless. They are closer to gods of, well, Chaos rather than Evil, being conglomerations of every mortal emotion; however, the relentless Crapsack World means they aren't fed with many good emotions, and those emotions are taken to such extremes anyway that they and their followers inevitably embrace madness, destruction and mayhem.
There are, however, some fairly unambiguously evil gods in Warhammer, although there is generally some argument (in-universe and out) as to whether they are gods in their own right, or aspects of another (and occasionally, as to which other god they are an aspect of). Khaine (God of Murder, Assassins and Poisoners) is a case in point, with some claiming he is an aspect of Khorne as worshipped by the elvesnote even to the point of claiming he's an elven blend of Khorne and Slaanesh, and the reason for the emnity of those two deities, while some say he's an aspect of Morr (God of Death, Dreams and Omens), and those who say Khaine is a god in his own right are split as to whether he's Morr's younger brother, or one of the children of Asuryan and Eldaneshnote Morr being a human god, while the others are part of the elven pantheon. Stromfels (God of Storms, Pirates, Sharks and Wreckers) is in a similar, though less extreme positionnote being either an aspect of, or rebellious younger brother to Manann, God of the Sea. The Horned Rat, though, is pretty much a pretty straightforward evil god, complete with his own species of evil anthropomorphic rats to worship him.
Khaine in 40K. He was the elven pantheon's God of Evil, but when Slaanesh killed the rest of the gods, only Khaine was able to stand up to him. While he lost the battle with Slaanesh and could not prevent the Chaos god's ascension, he remains alive and the Eldar still pray to him on the basis that he's a monster and a villain, but he's still their monster and villain. Ultimately, while Khaine is war-like, arrogant and relishes in killing, he remains an Eldar god and doesn't want to see the Eldar die out.
The two Ork Gods, Gork and Mork, from 40K may be this, or may not. They are the Gods of being "brutally kunnin'" and "kunnin'ly brutal," one hits you in the back while the other hits you when you're looking at them (though no Ork really is sure which is which) and they do like to murder, kill, and fight just as any Ork. They are rarely not actively attacking and fighting anyone besides each other.
If the definition of "god" is expanded a bit beyond the setting's use of the term, the Yozis, the Neverborn, and some behemoths all qualify.
The Ebon Dragon invented the concept of betrayal, and had a significant hand in creating both fate and free will because the potential suffering caused by the contradiction between the two amuses him; this was before he became a Yozi. However, the Ebon Dragon is more accurately the God of Shadow Archetypes, the Primal Nemesis. He defines himself as the Nemesis of Light, so he's Darkness. He defines himself as the Nemesis of Trust, so he's Betrayal. He defined himself as the Nemesis of the Four Virtures of Compassion, Conviction, Temperance, and Valor, so he's Cruel, Pragmatic, Unfettered, and Spineless. In fact, he's stated to not truly exist in metaphysical terms-without other beings to state what his evil is, he's nothing at all.
Magic The Gathering has the God of Phyrexia, Yawgmoth, known in several cultures of Dominaria as the Dark Lord, the Hidden One, and the Lord of Wastes. It took a massive effort and the lives of countless people, including several Planeswalkers, to stop him when he attempted to take over Dominaria and spread dominion over The Multiverse. While he is believed to be dead, nothing is ever certain with Yawgmoth, and rumors linger that he is still alive, biding his time as he recovers his strength.
While the Theros block (based on Classical Mythology) averts this with Erebos, it still has two downrightly evil gods: Phenax, the deity of deceit and lies, and Mogis, the god of bloodlust, savagery and canibalism.
Two flavours in The Dark Eye: Demons want to destroy creation, the strongest are dark shadows of the major gods. The Nameless God is a renegade god who wants to rule creation and doesn't shirk from anything imaginable (and unimaginable) to achieve that goal.
The Wyrm from Werewolf: The Apocalypse was essentially the cosmic embodiment of suffering, hatred, and corruption, keen to blight all of existence into a wasted hell hole. Ironically enough, originally he'd been the blessed end that comes to all things, but then something very bad happened.
The Maeljin from Werewolf: The Forsaken are the most literal examples, being the spirits of such family friendly concepts as lust and war.
Charnel from Sacrifice is a Card-Carrying Villain God of Evil who is Genre Savvy about how important his place is in the pantheon. Amusingly, Charnel is publicly the most fervent opponent of the game's Final Boss, Omnicidal Maniac Marduk, as he doesn't like competitors. It remains ambiguous just how much he knows about who's really responsible for summoning Marduk and whether his lines of reasoning for siccing you on the other gods are genuine or simply him using you to settle his old grudges.
Silent Hill is unhelpful as to whether the power of the eponymous town and the deity its cult attempts to summon is a God of Evil or an Eldritch Abomination. It could even very well be entirely neutral, with characters projecting their own evil onto it.
In Fire Emblem Tellius, the Goddess of Evil is also a Sealed Evil in a Can which will awaken when enough violence and fighting takes place. Until it turns out that she's really just the Goddess of Chaos, counterpart of the "good" Goddess of Order, and mankind cannot exist properly without both of them in balance.
Falerin of AdventureQuest is an interesting subversion; he is the God of Evil, but he is not malevolent and bloodthirsty. He's quite gentlemanly and passive; in fact, he helps the protagonists indirectly against the actual Big Bad.
The Daedric Princes of The Elder Scrolls are generally regarded as such by human society. Most of them are just very morally flexible with how they and their followers accomplish their goals—no Prince's domain is straightforward "evil". However, Mehrunes Dagon is the Prince of chaos and destruction, Molag Bal is the Prince of dominion and rape, and Vaermina is the Prince of fear and nightmares, so they more or less fit the trope to a T.
And within that set, Molag Bal is the closest there is to an actual God of Evil. While Mehrunes Dagon and Vaermina simply have domains with negative connotations (and Mehrunes has some with fairly neutral connotations, it's just that he favours the more negative ones), Molag Bal is also the god of corruption - he gets his power by making mortals do evil deeds. Essentially, he's the Elder Scrolls equivalent of Satan, particularly the older depictions of Satan when he was more of a corrupter of mankind than a general embodiment of evil.
Also Sithis is said to be a representation of one of the primordial states of chaos. He is the patron god of the Dark Brotherhood assassins. He is technically neither Daedra or Aedra. The Unofficial Elder Scrolls Wiki has a full writeup, based on all the references to "him" in the games.
The Elder God in Legacy of Kain is eventually revealed to be this in Deception, following truckloads of Foreshadowing in Soul Reaver 2.
Tathamet, the Prime Evil and God of Chaos in the Diablo verse. When Tathamet suffered a Mutual Kill with Anu the God of Order, his body became Hell and his seven heads became the Seven Great Evils, including series Big Bad Diablo. In Diablo III, it's revealed that Diablo's scheme all along was to reunite the Great Evils into Tathamet reborn (with Diablo at the helm) to end the war between Heaven and Hell once and for all.
The Painkiller series started out with Satan in the role, but he was killed in the first game, and a veritable conga line of successors would pop up in the expansion packs one after the other to claim the title (first Alastor, then Eve, then Samael).
Skorm from Fable, who the PC can make human sacrifices to. Doesn't actually exist. More important than Skorm is the Corruption, the setting's Bigger Bad, as well as the Queen of Blades, who had the position in the setting's backstory.
In EarthBound, many hints given throughout the game indicate that Giygas has essentially become this. Porky's statement during the final battle even says that he is evil itself.
Bethrezen and Mortis in the Disciples series. Bethrezen is the god the Legions of the Damned worship and Mortis is the goddess of The Undead Hordes. Bizarrely, they are also the most sympathetic deities in the setting. The "good" gods are a bunch of Jerkass Gods whose cruelty towards Bethrezen and Mortis drove them to evil in the first place.
Bethrezen is the creator of Nevendaar, the world where everything takes place but was not a deity in his own right, being instead an angel of the elder god Highfather. Both Bethrezen and Mortis are Mad Gods, as is Gallean.
In Age of Mythology, Greek, Egyptian and Norse mythology are simplified somewhat from the original myths for the sake of the campaigns. As a result, Set becomes this for the Egyptian, Loki for the Norse (especially notable for turning up as an antagonist in person, and serving a much more direct role than Set), and Poseidon for the Greek. However, the ultimate God of Evil is Kronos, from the Atlanteans, since the other three are ultimately subservient to him, and his master scheme of breaking loose and causing The End of the World as We Know It. All the Titans except Gaia also qualifies, while Cerberus and Ymir are more Eldritch Abominations anyway. The game is notable for averting Everybody Hates Hades though, Hades is somewhat neutral, and his only contribution is helping the heroes escape Erebus when they are trapped there by Gargarensis, the servant of Kronos.
Played with in Fate/stay night. During the Third Holy Grail War the Einzberns tried to guarantee a victory by summoning the Zoroastrian god of evil, Angra Mainyu. What they in fact summoned was a man whose village had declared him to be the source of all their sins, his existence being the origin of Angra Mainyu's myth. Having no special abilities he was quickly killed and absorbed by the Grail. But because this man represented the wish of those ancient villagers for there to exist a singular source of evil, the Grail attempted to grant that wish. Had there been a victor in either the Fourth or Fifth Grail Wars, he would have become a Physical God.
The Demon King in Sluggy Freelance is worshiped by his demon followers and was powerful enough to defeat the Goddess of Goodness. He even sealed her away in a plastic baggie, which he kept in his refrigerator.
Due to the D&D-style setting, there exist various gods belonging to different pantheons, some of which are deemed "evil" gods. Despite this they seem to get along well enough with their good counterparts (as shown by the interactions between Loki and Thor, and in some prequels) with the worst clashes being arguments over what to do in certain circumstances... and for good measure the "good" gods aren't always Genre Savvy.
The Transformers franchise has the Dark God Unicron, the Chaos Bringer and arch enemy of the Creator-God Primus.
Tutenstein has one but there's no time to explain.
Trigon from Teen Titans who, like his original comic book counterpart, is supposed to be the incarnation of all evil.
In My Little Pony: Friendship Is Magic, the opposed good and evil general principles are presented as being harmony and chaos. Discord is the spirit of chaos and disharmony and a Reality Warper at Physical God levels. In the distant past, he ruled the entire country in a state of perpetual unrest and unhappiness. Two whole seasons since his debut, he's still the most powerful entity encountered in the series by a wide margin.note Word of God has apparently hinted at something that may be a Bigger Bad even beyond him, but this may no longer count for anything since Lauren Faust is no longer working on the series and, judging from what she said, she didn't leave them a list of ideas to use later.