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* In ''TabletopGame/BattleTech'', the [[ChurchMilitant Word of Blake]] violates centuries-old rules of warfare forbidding Weapons of Mass Destruction from being used on civilians. All the other factions within the Inner Sphere are so horrified that they [[EnemyMine joined forces]] to annihilate the Blakists... [[PayEvilUntoEvil with even more Weapons of Mass Destruction]].
** Stefan Amaris is perhaps the only person in-universe who has been directly compared to UsefulNotes/AdolfHitler... in fact, it wouldn't be surprising if a 3025 version of GodwinsLaw actually named Amaris. His crossing of the horizon came when he [[RulingFamilyMassacre executed the entirety of the Cameron family]], and then left their bodies to rot in the throne room where they were executed.
** Another MEH was crossed by Jinjiro Kurita when he massacred almost the entire population of Kentares IV.
** During the Clan Invasion, Clan Smoke Jaguar ordered an orbital bombardment over a city full of civilians; this act enraged the Inner Sphere and [[EvenEvilHasStandards the other Clans called it needlessly cruel]]. [[LaserGuidedKarma As a result, when the Inner Sphere united to strike back against the Clans, Smoke Jaguar was the one they singled out for total annihilation.]]
* ''TabletopGame/{{Exalted}}'': Being a loyalist Abyssal or an Infernal counts as -- or requires -- crossing the Moral Event Horizon. Signing up with the [[OmnicidalManiac Neverborn and the Deathlords]] means voluntarily killing absolutely everything; your family, crush, dreams, and hope included. Hopping on the [[HellOnEarth Reclamation]] wagon means that you will devote your godlike powers to freeing infinitely hateful demon-gods upon the world, who might as well rewrite reality so that nothing can die and everything suffers eternally. Still, this being Exalted, this doesn't describe all of them, [[NobleDemon or even most]], particularly in the case of the latter. The Abyssals may repent and reclaim their original nature of the Solar Exalted. A possibility is convincing their good nature to the Unconquered Sun, who is now sadly addicted to Celstial Crack and has not paid attention to Creation in a thousand years. Another is reaching Autochthon (who is hiding in a different plane of reality) and talking him into rewriting the corrupted Exaltation. Nobody has achieved either of this... ''yet'' (although, to be fair, the first Abyssals were created only a few years back at the time of the setting's default chronological campaign starting point). For an Infernal, stealing away the powers of the Yozis, subverting their control and becoming a proto-Primordial on their own can allow them to pursue their own heroic, not-necessarily-sadistic goals. This is about as difficult as inventing a new type of reality. [[ShowyInvincibleHero Which is to say]], [[GodModeSue "not the most"]]. These ''are'' people meant to remake reality, not destroy it, after all.
* In the TabletopGame/NewWorldOfDarkness, every person is subject to a zero-to-ten KarmaMeter, with an average person falling in somewhere around seven. For normal humans, falling to a low Morality only gives a penalty in that getting to that level requires committing acts that would get you tried under the Geneva Conventions, if not thrown in prison for life (though one is unlikely to drop very low without going at least a little insane). For supernaturals, however, falling down on their respective morality scales often imposes supernatural penalties, and for both regular humans and all supernaturals, falling to 0 irreversibly corrupts you in some manner and you're turned into an NPC. The "irreversible corruption" works in different ways:
** [[TabletopGame/VampireTheRequiem Vampires]] become ''Wights'', ravening blood-crazed horrors lost to their inner beast.
** [[TabletopGame/WerewolfTheForsaken Werewolves]] become obsessive-compulsive [[ImAHumanitarian cannibals]] called ''Zi'ir'', or the Lost.
** [[TabletopGame/HunterTheVigil Hunters]], regular humans, and minor Supernaturals become sociopaths incapable of meaningfully interacting with others.
** [[TabletopGame/PrometheanTheCreated Prometheans]] become the monsters they resemble.
** [[TabletopGame/GeistTheSinEaters Sin-Eaters]] have their minds and souls shattered, with their Geist as much or more in control than they are of the pitiful wreck they've become.
** [[TabletopGame/ChangelingTheLost Changelings]] with Clarity 0 have a bad habit of disappearing into the Hedge and never coming back. Why? [[spoiler:Because they become [[EldritchAbomination True Fae]] themselves]].
** [[TabletopGame/MageTheAwakening Mages]] become ''The Mad'' and are, as the name implies, completely insane. They range from "merely" insane mages, the equivalent of autistic savants of magic, to utterly transformed beings -- bizarre and twisted constructs of flesh, minds freed from their bodies, forces of nature, or supernatural phenomena. That's at ''best''.
** Geniuses, from the fanmade ''TabletopGame/GeniusTheTransgression'', have to essentially be Mengele to reach these lower points. When they hit zero, their individual personality largely disintegrates, and they become monsters that won't give a damn how many people they have to hurt in order to complete their experiments.
** Princesses, from the fanmade ''TabletopGame/PrincessTheHopeful'' tend to become worse and worse the more they compromise their Beliefs. If they lose all Belief and they do not manage to escape for their next reincarnation, they become [[TorturedMonster Dethroned]], whose overpowering emotions of despair warps anything touched with Darkness (including the DarkWorld) and are more than happy (if they could even feel the concept of happiness or hope) to wallow away. They're not completely inactive, as they'll lash out at anything that reminds them of what brought them to their state. This is one of the few [[SubvertedTrope subversions]] for [[MoralEventHorizon this trope]], as when a Dethroned is slain, any other Princess can take the Dethroned's grief into themselves and redeem/release the fallen Princess from the Darkness to continue living through the rest of their reincarnations.
** Dragons from the fanmade ''TabletopGame/DragonTheEmbers'' usually have to take part in torture (which is the one line even the vilest of them usually are reluctant to cross) to hit 0 on their Ethic. This results in them becoming Hydras, nine-headed mindless monsters who go around wreaking havoc and devouring everything in their path.
** Leviathans from the fanmade ''TabletopGame/LeviathanTheTempest'' become Typhons, vicious beasts unable to return to their human form who are only driven by their most primal urges to mate, dominate and protect their territory.
** Mortals and Hunters are the only groups who actually follows what anyone would really call morals, even if there's noticeable overlap, because the supernaturals have already shed their human viewpoints, and different acts ding different numbers on different meters. Vampires are essentially fighting off mindless, bestial gluttony, while Mages have to resist succumbing to [[AGodAmI their own egotism]], for instance. The books tend to recommend varying levels of harshness from storytellers if a particular act doesn't hit a listed threshold, but illustrates a noticeable move toward the lower end of the scale. The scales aren't balanced across each other, either. Sin-Eaters, for instance, can lose permanent ranks of Synergy, while a Changeling is never permanently barred from reaching Clarity 10, but a Changeling's clarity ''[[BeingGoodSucks never]]'' [[BeingGoodSucks makes exceptions for motivation or extenuating circumstances]] when considering dings on the meter; it might mean penalties or bonuses if the ST feels it's appropriate, but a degeneration roll is always made regardless.
* In the ''TabletopGame/OldWorldOfDarkness'' system, vampires who wanted to start down an alternate path of morality instead of the default humanity had to commit some atrocity in order to shed their humanity forever. Once this was done, they became true monsters who couldn't even pass for human. However, whatever atrocity the vampire commits isn't what pushes them over the MEH; it's the resulting rejection of humanity that the atrocity symbolized.
** A vampire which follows one of these alternate Paths can make for an interesting discussion on whether or not they can truly committ a moral event horizon. By following such a Path, they adhere to a completely new and alien system of morality not easily understood by a vast majority of others. When such a being committs a [=MEH=], it may seem an atrocity to others but perfectly acceptable to their own moral guidelines.
** It's possible to commit the atrocity and still fail to switch over their morality. The most likely result is a complete loss of sanity.
** Likewise, in ''TabletopGame/MageTheAscension'', you have the Nephandi, mages who have sold out their souls to dark and terrible masters in the name of universal destruction. To do so requires the mage go through a Caul and turn their Avatar (the representation of their higher magical beliefs) inside out. This is something few mages do accidentally; to truly become a Nephandus requires ''knowingly'' desecrating everything you hold dear, from your faith to your relationships to your morals.
* The GothicHorror setting of ''TabletopGame/{{Ravenloft}}'' for TabletopGame/DungeonsAndDragons has an official term for one of these: an [[NamesToRunAwayFromReallyFast "Act of Ultimate Darkness,"]] which is a requirement for becoming one of the setting's dreaded Darklords -- an act of such great and horrible evil that it is guaranteed to draw the attention of the Dark Powers, and is a near-perfect blend of {{hypocrisy}}, [[ThisIsUnforgivable depravity]], [[KickTheDog cruelty]], and [[ItsAllAboutMe selfishness]]. The clincher, though, [[ObliviouslyEvil is absolute refusal to acknowledge that what they did was wrong]]. Indeed, that's part of ThePunishment for Darklords -- that if they worked up the moral strength to admit that what they have done is inexcusable and that they reaped what they sowed, their curse would be [[CursedWithAwesome moot]]. Then again, the books say that if they were the sorts of people who'd be able to do that, they would never have become Darklords in the first place. Notable examples include:
** Probably the most famous Act of Ultimate Darkness was committed by Count Strahd Von Zarovich, who rules the domain of Ravenloft. He not only made a deal with darkness to become a vampire, but he murdered his brother Sergei over Tatyana, the woman both men loved, on their wedding day, leading to Tatyana throwing herself off the wall of Ravenloft as Strahd pursued her. Every generation, Tatyana is reincarnated and Strahd pursues her to her death, never learning his lesson.
** Lord Soth, originally from [[TabletopGame/{{Dragonlance}} Krynn]] and ruler of the domain of Sithicus, committed several major acts that would qualify as Acts of Ultimate Darkness:
*** He and his first wife, Lady Korrine of Gladria, had been trying to produce a son to be his heir, and Korrine had consulted a witch about the problem, who had agreed to help them, but had warned her that the child would be a representation of Soth's soul. Unfortunately, Korrine didn't know about the evil deeds that her husband had done, including ordering the murders of his half-brother and sister by his seneschal Caradoc, else she would have known what would eventually transpire of the birth and would be of a mind to curse the witch. When she gave birth to the son in question, it had a face similar to that of dragon-kin with two arms on one side and a leg on the other, with the last leg placed at the bottom of the buttocks as if it were a tail. To say that Soth was pissed about this was a massive understatement and, thinking that she had cheated on him with some kind of demon, Soth murdered both Korrine and the monstrous child.
*** After marrying a second wife named Isolde, he set out on a quest to stop the Kingpriest from unleashing the Cataclysm upon Krynn by forcing the Rod of Omniscient Wisdom into his hands (according to Isolde's vision, it would take many tries, and each time he was killed, he'd rise with greater power) in return for redemption. When Soth and the thirteen knights with him found the Rod, he left his soul due to the curse on the coffer, becoming a type of {{Lich}}, with his soul residing in the coffer like a phylactery, astrally projecting into his body, and unaware of this new state. On his way to Istar, he came across three elf-maids who proceeded to poison him against Isolde, telling him lies about her infidelity and saying that she had sent him on this quest to die in order to get rid of him. Soth got pissed again, returned home, and confronted his wife just as the Cataclysm began. A chandelier fell on Isolde and their newborn son, and she begged for him to save their son, but Soth stopped himself from doing so, so as to prevent his own son from growing up as he himself had. [[DyingCurse With her final breath]], Isolde cursed him to live the lifetime of every soul that he had caused death on that day, and as Soth's keep burned down, Soth became a death knight and his retainers became undead.
*** Interestingly, Soth was eventually freed from Ravenloft when he fell in a catatonic state after he hit the DespairEventHorizon (implied to be due, in part, to [[HeelRealisation realising/accepting how everything that had gone wrong in his life and unlife was his own damn fault]]), as [[ThePowersThatBe the Dark Powers]] saw that whatever else they did, they couldn't make his existence any more wretched than it was. The {{Doylist}} explanation is that the company that had the license for Ravenloft for most of Third Edition, Creator/WhiteWolf, did not get the rights to other campaign settings (Lord Soth's from Literature/{{Dragonlance}}) with it, and so they had to do away with him in a manner that did not break the law or the setting. That, and that Soth's creators had been in a major ArmedWithCanon battle over another Creator/{{TSR}} writer putting Soth in Ravenloft in the first place, so the last thing they wanted to do was rouse that dragon again.
** Azalin Rex, ruler of the domain of Darkon, executed his own son after catching him freeing political prisoners.
** Lord Wilfred Godefroy, ruler of the domain of Mordent, murdered his wife and daughter with his walking stick because his wife hadn't given him the son he wanted.
** Meistersinger Harkon Lukas, who rules the domain of Kartakass, abused his position as "Grandfather Wolf" in order to bring civilization to his homelands, driving out [[OurWerewolvesAreDifferent his own people]] in the process. Interestingly, the Act wasn't enough to catapult him to Darklord-dom; rather, it was using the colonists [[ImAHumanitarian as a food source]], which isn't normally a powers-check-worthy act for wolfweres, as well as the betrayal of trust.
* In ''TabletopGame/{{Shadowrun}}'', the Horizon MegaCorp in ''Columbian Subterfuge'' reveals they killed [=POWs=] rescued by the players' shadowrunners and made it look like Aztlan did it.
* Happens every two seconds in ''TabletopGame/{{Warhammer 40000}}''. Half of the factions don't care about the Moral Event Horizon, and the other half [[StarfishAliens don't even know it exists.]] [[CrapsackWorld Yeah, that's not good.]]
** Horus went over the line when he destroyed Isstvan III in an attempt to eliminate the loyalist elements of several Legions. This included members of his own Legion, who looked up to him like a father. When some of them dug in and survived the virus-bombing, Horus ordered the forces loyal to him to go down to the surface and kill them, making them complicit in his treachery and betrayal. For the Emperor, however, it was flaying Ollanius Pius (who is either an Adeptus Custodes, an Imperial Fists Terminator, or an ordinary Guardsman: [[FlipFlopOfGod Games Workshop can't decide which]]) alive with mind bullets that was what finally convinced him Horus was beyond saving.
** In-universe, Tau view the killing of an Ethereal as a crossing of the MoralEventHorizon. If you do this, they either enter a state not at all different from a HeroicBSOD, or they go ''[[RoaringRampageOfRevenge absolutely]] [[UnstoppableRage nuts]]'' and proceed to ruthlessly massacre the enemy army with an [[BeamSpam unrelenting tidal wave of plasma fire]].
** Also in-universe, Eldar view destroying an Eldar soulstone as crossing the MoralEventHorizon, as anyone who does submits the Eldar soul inside the stone to a FateWorseThanDeath at the hands of [[GodOfEvil Slaanesh]].
** In ''TabletopGame/DarkHeresy'', you can spend experience to remove corruption, but you can't reduce your corruption level (which goes up every 10 corruption points) and thus can never remove malignancies. At the highest level of Corruption -- 100 points -- a character is "Damned" and is functionally killed off... unless, of course, you bring in ''TabletopGame/BlackCrusade'', which makes 100 Corruption just the beginning.
* The murder of the beautiful cleric Aleena with a Magic Missile by Bargle the Infamous from the introductory adventure of Basic D&D would make the evil magic user an enemy for life for many a player back in the 80s. (Of course, as far as villains go in the big scheme of things, Bargle was rather tame... ''But'' he had the ''potential'' to become a rather vile villain; it all depended on just how far the DM was willing to take the character.)