Oersted in Live A Live. After being tricked, he finds that everyone has now abandoned him and considers him a demon, his only remaining ally is dragged away to be tortured, and is blamed for the death of said ally who expends the last of his power to set Oersted free. Oh, then he finds out that his best friend betrayed him to this fate because he was jealous. Oh, and the 'Aesop' which has been so far in the game? "Don't lose hope as long as somebody believes in you". That went well. The last person who he hoped believed in him, the princess? After Oersted duels his traitorous friend and kills him, she asks why he didn't come to rescue her (ouch. He did. Straybow only got there first by faking his death and ruining Oersted's life), declares that she loves said traitor, and kills herself. That was the absolute last straw, the severing of his last tenuous tie to sanity. The result? DemonKingOdio is (re)born and is bent on destroying humanity.
After spending a year in coma and seeing the destruction of the world firsthand, Cid's death in Final Fantasy VI proves to be Celes' final straw, driving her to toss herself off a cliff. She survives by a miracle, and seeing Locke's bandanna tied around a pigeon's wing gives her a new reason to live.This event can be prevented by successfully playing a minigame, but the path of failure is much better written.note The developers seem to think so too, because the 'right thing' to do is willfully obscure, and the remainder of the plot tacitly assumes the character is dead.
Several other characters get dangerously close as well. Strago, believing Relm is dead, allows himself to be brainwashed by the Cult of Kefka, although he snaps out of it once he sees that Relm is alive. Setzer, after losing his beloved airship, is found drinking at the bar in Kohlingen, and it takes a Rousing Speech from Celes to bring him around.
This is Seymour's motive in Final Fantasy X. After a fairly crappy childhood he hits when his mother (the only person who ever loved him) sacrifices her own life to give him the power to defeat Sin. His despair drives him to plot the destruction of all life in Spira because he sees it as the only way to bring an end to all suffering.
It also turns out to be the motive for why Yu Yevon originally created Sin a thousand years ago. Seeing that his beloved city of Zanarkand would be destroyed, he killed every living being in it and used their souls to create an eternal Dream Zanarkand, as well as an all-powerful destructive force (Sin) to provide the power needed to keep Dream Zanarkand alive.
And in the sequel Final Fantasy X-2, a Despair Event Horizon is the main motive for its Big Bad, Shuyin, who wants to destroy the world which let him and his beloved die a thousand years ago. To put it in perspective, Shuyin's concentrated despair festered in a hole for a millenia after his death until it reached a point where the only way he could think to end his pain would be to end the world itself.
It isn't clearly shown onscreen, but the backstory of Tales of Symphonia makes it apparent that Mithos, Kratos and Yuan all suffered this upon the death of Martel, Mithos' sister and Yuan's fiancee, leading the heroes of the ancient world to become villains instead.
Kratos arguably claws his way back out of the Despair Event Horizon when he becomes disillusioned and meets his lover, Lloyd's mother, falls back in when he thinks they're both dead, and then claws back out again when he encounters Lloyd later.
Zelos spends much of the game near this line, despite hiding it, as his terrible childhood and lonely life gave him little self worth and no friends. Lloyd's friendship and trust helps him recover...until Flanoir where Lloyd either confirms his trust or questions it. The latter sends Zelos over the Despair Event Horizon, and he later commits suicide by cop.
The sequel has Alice fall into this when Decus is killed protecting her.
The second half of Leon's story in the remake of Tales of Destiny is all about breaking Leon down past this point until he ends up betraying his friends. Only his Heroic Sacrifice gives him any solace. In the original, his death was purely villainous, and he had to wait until the sequel for any sort of redemption. He does get better, though.
Final Fantasy XIII has each of the main characters going over the DEH. Some are more destructive than others.
Lightning, in frustration with PSICOM's persistence in hunting down the L'Cie and Hope's ineptitude finally goes over the edge. After what could be considered a "The Reason You Suck" Speech, she inadvertently summons her Eidolon, Odin, who wastes absolutely no time in attacking Hope. What gives this example a more literal sense is noting that as long as Hope has health remaining, Odin will always target him first in battle.
Hope has a number of these. The first starts as a mere lack of will to live. Nothing too major. Then, he gets pushed into a state of Tranquil Fury against Snow after the man saves him, believing he didn't save his mother, marking Snow as irresponsible. Needless to say, Hope and Lightning begin to bond due to their mutual dislike of the man, with the latter unintentionally giving Hope the will to create a premeditated murder plan. Just to add to the blunder, she also give him a Survival Knife. This marks the longest in-game grudge, spanning for at least four chapters. Eventually, he comes to a Rage Breaking Point after fighting a boss, which causes him to explode with anger, blasting Snow off of a roof, leaving him hanging over the edge while Hope attempts to kill him with the same knife given to him. However, this alerts the PSICOM forces looking for them, blast Hope from the roof. This, however, is passed once the man saves his life. His final, albeit short, one is experienced on the surface of Gran Pulse; where Hope finds that the powerful forces of Pulse are too much for him. He goes on a Heroic Self-Deprecation rant, which unlocks his Eidolon, revealing the truth that Eidolons come when you are on your last legs to display your inner strength to you.
Sazh spends a good bit of his time with Vanille over the deep end. However, he has one of the more memorable ones in the game. After travelling with the girl, soon becoming rather attached to her; Sazh is confronted by Commander Jihl Nabaat, the woman who was to watch over his Cocoon l'Cie-branded son. The woman then reveals that Vanille was the reason Dajh was branded in the first place, and thus the reason he ended up as a crystal, which causes the girl to run in sadness of the revelation. And Sazh to go after her in a Roaring Rampage of Revenge. Soon, he succumbs to the sadness of his son's fate, opting to let his then summoned Eidolon, Brynhildr, kill him. After her defeat, he decides to attempt to commit suicide by pointing his own gun to his temple.
Snow has a large one after the first battle with the penultimate boss, who revealed to the group that Serah's Focus was to destroy Cocoon, instead of saving it. Snow becomes nearly catatonic, with both Vanille and Lightning having to help him out of his rut and restore his faith in Serah. Afterwards, Snow's brand was the farthest along to turning him into a Cie'th; as emotional distress increases the speed of the process. Both of these are lampshaded by Fang.
Vanille suffers a few of them. A famous one is her breakdown once Fang "remembered" everything, exposing Vanille's lies and her Eidolon. This is pushed to a climax after the battle, where Fang motions to knock Vanille a new one... only to stop and hug her, allowing the girl to cry into her shoulder.
Fang suffers two of these in-game. The first is shown when she summons her Eidolon, Bahamut, accidentally after her tirade against the Fantastic Racism Coccon is capable of. Opting to "blast Cocoon out of the sky", she very briefly turns on the group; even drawing her weapon out at them. She suffers a much more emotional, but brief, one after her friends are turned into Cie'th by Orphan's illusion.
Snow, over Serah's death, spends almost the entire game in one, which results in him becoming a Cie'thin hopes of Lightning killing him. Fortunately, Lightning is able to pull him back from the Horizon.
Noel, due to the aforementioned death of Serah and his role in the death of Etro, suffers from immense guilt, leading to him chasing a prophesy of him killing Lightning in hopes of reuniting with Yeul.
Even Caius Ballad has crossed the DEH. After killing Etro, he no longer had a purpose and longed to dieóbut while some of the incarnations of Yeul wished for his suffering to end, others did not want him to die and revive him each time he does, leaving him unable to die and with no purpose.
Lumina crosses it near the end of the game, as because she is a manifestation of Lightning's emotions that was sealed away after Serah's death, she knows that if Lightning does not accept her, she'll vanish forever.
In the ending, Lightning crosses it when she realizes that sealing herself with Bhunivelze in the Unseen Realm means that she'll never be with her friends again. Then Lumina arrives, and once Lightning accepts her emotions, is able to call out for help.
In Fallout, it is possible to send the Master over the Despair Event Horizon by convincing him that his plan to conquer humanity with his Super Mutants is doomed to failure.
"But it cannot be! This would mean that all my work has been for nothing! Everything that I've tried to...a failure! It can't be!Be!Be! Be...I...don't think that I can continue.Continue? To have done the things I have done...in the name of progress and healing...It was madness. I can see it now, madness. Madness? There is no hope. Leave now. Leave...while you still have hope..."
Tales of the World: Radiant Mythology has Kanonno, after her home world, Pasca, was consumed by war and mana abuse despite her efforts as its Descender. She even committed genocide, which led to Pasca's rapid decline.
Devil Survivor's Yasuyuki Honda crosses this in the Escape ending. He spends the whole week trying to escape the lockdown so he can see his hospitalized son, only to discover he's too late, and because they broke through the blockade, the whole world's gone to hell. This sends him completely over the edge, and he calls out the 'heroes' responsible in a nightmarishly backwards way, complete with Slasher Smile.
Earlier on, Keisuke, already destabilized by a whole childhood of witnessing and suffering bullying, finally snaps when he sees Midori being nearly lynched to death to the people she was trying to save. He spends the following days going Knight Templar and killing anyone that commits any kind of injustice.
The opening of the first God of War has Kratos say the gods have abandoned him and he throws himself off a cliff. The rest of the game is spent showing what pushed him over the DEH while he slaughters his way through a Greek mythology textbook.
This is the entire premise behind Knights of the Old Republic II, as Revan is revealed to have deliberately subjected his Jedi Knights to brutal, dehumanizing battles until their spirit breaks and they turn to the dark side.
Before Digital Devil Saga starts, Angel crossed this when her boyfriend died in her arms from injuries sustained in a terrorist attack on his hospital, all due to fear of the new, possibly not even infectious disease she'd been working on finding a cure for.
Ryu's Despair Event Horizon occurs when the summoners of Chek (who all appear to be children) are massacred in front of him by Rasso. Ryu ends up having a Heroic B.S.O.D. which releases the Kaiser Dragon—a kaiju-esque dragon that is a Physical God—and results in him going into an Unstoppable Rage until receiving a Cool Down Hug from Nina.
This is also after no less than two previous attempts by The Empire to kill Fou-lu. Who is a literalGod-Emperor. Whom the Vestigial Empire predecessors of The Empireexplicitly summoned there 600 years previously (botching the summoning and leaving the Physical God they were trying to pull across in two separate bodies separated by 600 years on opposite sides of the world) to unite their people and bring peace. And the main person trying to kill him, Yohm, is also explicitly a priest who knows damn well who and what Fou-lu is but is trying to kill him anyways.
Hunter, in Darkness, an interactive fiction by Andrew Plotkin, plays this one and plays it hard. An early segment has you crawling, cut and bleeding, through a tight space with hundreds of feet of rock above and miles and miles of rock below. The game assures you repeatedly, even as you struggle ever closer to freedom, that this is it—your final resting place. This is where you die. Your light has flickered out and the invisible narrator insists that you will never leave this place, until finally you tear yourself away from the stone gullet and stumble blindly to freedom.
Wild ARMs 2: A certain Eldritch Abomination was sealed away "beyond the Event Horizon" in the backstory, and happens to be the source of the protagonist's Super Mode. Said protagonist is pushed to the edge of the Despair Event Horizon midway through, and teeters there for the entire game. After defeating the supposed final boss which involved his Magnificent Bastard commander sacrificing himself to give an Eldritch Location a physical body to kill, he suffers a Heroic B.S.O.D. that almost releases the demon. In other words, his despair formed a bridge across the Event Horizon.
Dwarf Fortress has this as a fundamental aspect of managing your dwarves. They can become unhappy through a variety of reasons from simply not having any alcohol to drink through to the deaths of close friends or family. Getting a dwarf depressed enough will push them over the edge and cause them to lose their sanity. Depending on several factors this can be anything from a minor nuisance to extremely dangerousTo be specific... Depending on the dwarf's personality, they can either tear off their clothes and run all around the fortress babbling madly, simply stop responding to the world and stare off into space for the rest of their now-short lives, commit suicide through water, magma or starvation, or trying to kill everything that moves. If you're lucky, events can contrive to make this game-ending depending on your fortress design and how close other dwarves are to being over the edge themselves. Indeed, events can (or more usually, do) spiral out of control until your entire fortress is over the Despair Event Horizon.
Hiroki in Canvas 2 in regards to painting. Many of the routes actually deal more with helping him get over his problems than helping the heroine.
In Dragon Age II, the player can push Fenris over the Despair Event Horizon by agreeing to give him back to the blood mage who had previously enslaved him. He's so gutted by the betrayal that he doesn't even get angry, just bows his head and leaves with the slaver without a fight.
Implied with Orsino at the end of the game. Seeing his fellow mages cut down one by one with no hope of surrender, he finally gives in and uses blood magic in a 'taking you with me' type deal. If you're *on* the side massacring the mages, he's downright Laughing Mad as he gives in. If you're helping the mages, the casualties are still too much for him. "Why don't they just drown us as infants? Why wait?"
The Elder Scrolls IV: Oblivion has Agronak gro-Malog, the Orcish Grand Champion of the Arena. His father was a noble, and he asks the player to help him find the truth of his heritage. After the player reveals to him that his father was a vampire, he believes himself to be a monster and doesn't even fight back when the player fights him in the Arena.
In Fire Emblem: Mystery of the Emblem, Prince Hardin of Aurelis, now Emperor Hardin crossed the horizon when he learned that while Princess Nyna cared for him, she didn't romantically love him and only agreed to marry him for the sake of Archanea, This led him to locking himself in his personal room alone drowning his sorrows in booze, until Gharnef disguised as a merchant offered him the Darksphere, and turned him into a megalomaniacal tyrant, setting off the events of Book 2 with Hardin himself as the Big Bad. His Famous Last Words are pretty much a Final Speech in which he begs Marth for forgiveness and begs him to apologize to Nyna in his stead for having been weak enough to give into the corruption of the Darksphere, finishing with his reaffirmation of his love for Nyna.
Fire Emblem: Genealogy of the Holy War: Lord Sigurd can hold himself up as well as possible when his wife Dierdre is kidnapped, his father Byron is falsely accused of treason and dies, he himself is accused of treason, and both his sister Ethlyn and her husband/Sigurd's Lancer Quan are murdered. However, the moment he sees that his kidnapped wife was actually mindwiped into marrying Arvis, who once was his sort-of ally, he can only scream in utter horror and betrayal right before Arvis himself burns him to death, and Sigurd's army is decimated shortly afterwards.
Sigurd's old friend and companion Tailtiu doesn't fare much better. In the second part of the game we learn that she crossed the DEH after not only being separated from her son Arthur as she and her daughter Tine were kidnapped back into Freege, but also after being subjected to endless abuse from her sister-in-law Hilda, which finally drove her to sickness and death.
Fire Emblem: Thracia 776: Eyvel once was close to this horizon. So much that whatever happened to her ( the Basttle of Barhera), robbed her not only of her husband and children, but of her memories of having ever been Briggid, the lost princess of Jungby and a member of Sigurd's troops. Much to our relief, at the end of the game we learn that some years after the second war of Grandbell, she recovered her memories and reunited with Patty and Faval.
Olwen and Misha also were pretty close to it, once they learned about the horrifying child hunts conducted by the Grandbellian empire. They both pulled through it via Heel Face Turns.
Fire Emblem: The Binding Blade: The biggest example is King Mordred of Etruria, who completely lost the will to reign and left the government to his advisors (many of them treacherous) after his son and heir, Prince Mildain, died in an accident. And then this is subverted in the end: Mildain actually reappears, having survived the ambush that almost killed him, and is reunited with his father, so we know that Mordred ultimately got better.
Zephiel Used to Be a Sweet Kid before he crossed his own Despair Event Horizon as well. Throughout his life he had been trying to please his father, but in vain. But when his father started trying to poison him, not only did he give up trying to please his father; he lost all faith in humanity as well. This is further detailed in The Blazing Blade, where you save him from an assassination attempt orchestrated by said father.
Fire Emblem: The Blazing Blade: Eliwood and Nils both come dangerously close to crossing this, once Eliwood is tricked by Nergal into killing Nils's beloved sister Ninian with Durandal when she is forced into her dragon form by Nergal himself.
Renaultdid cross it several years ago, when his partner and best friend was killed in battle. He was so utterly shattered that he threw himself into battle blindly (and killed Lucius's dad in the process) and collaborated with Nergal in hopes to have his friend revived, among other things. He recovered a part of his sanity, however, and became The Atoner in the end.
And then we have Harken, an Ace who was throughlybroken after being the Sole Survivor of Lord Elbert's decimated crew. We meet him as a powerful enemy whom the Pheraen characters must recruit as soon as possible lest he fights them to death, and his supports reveal how the horrors he witnessed and the helplessness he felt drove him into becoming an Empty Shell of the man he once was. For worse, some other supports (specifically, with Marcus) heavily imply that he had severe self-esteem and abuse/abandonment issues before the whole Break the Cutie deal.
When we meet Lord Hausen, the old man has crossed the DEH since he has learned that his long-lost daughter (Lyn's mother) is dead, his health is failing more and more (and he's being poisoned), and his brother Lundgren is taunting him endlessly about all of it. He's going to succumb to Death by Despair by that point. But when Lyn reaches for him, he manages to come back.
Fire Emblem: The Sacred Stones: Prince Lyon never was the most self-assured person, despite his Badass Bookworm status and being the Imperial Prince of Grado. (His weak health didn't help either.) But then his beloved father and idol Emperor Vigarde died of illness, so the poor guy's mental health took a BIG nosedive.
Then Lyon's advisor, Knoll actually managed to return from the DEH, having witnessed Lyon's fall into despair and then having been slated for execution. Ephraim and Duessel reach for him in the nick of time, and Knoll decides to join them and do what he can to help save Grado.
In Fire Emblem Fates, the Birthright path has Prince Xander crossing this on screen, as in the middle of his duel with the Avatar he accidentally cuts down his beloved half-sister Elise, who has taken a fatal hit meant for the Avatar him/herself, and she dies in his arms, and ends up becoming a Death Seeker as he fights the Avatar to the death. It even reflects in the gameplay: Xander is so guilt-ridden by Elise's death that he will never attack the Avatar unless the Avatar themselves instigates the battle, and even then doesn't do any damage when striking back.
Persona 5 brings this up as a gameplay mechanic. Anyone hit with the Despair ailment will skip their turn and lose a chunk of SP. If not cured within two of their actions, they will succumb to the ailment and die on their third action. During flu season (11/13 to 11/15, then 12/8 and 12/9) random enemies in Mementos will be afflicted with this ailment, and The Reaper itself is no exception.
In Orthodox traditions, Despair (or Tristitia in Latin) is considered to be an eighth Deadly Sin, and the major villains of Persona 5 embody the trope. In this instance, Tristitia, or Cavum ("Emptiness"), is represented by Goro Akechi, the charismatic detective who is revealed to have the protagonist arrested as shown at the beginning of the game. His horrid upbringing as a result of being a Japanese bastard child birthed a profound loneliness and anger that ultimately drives him towards revenge against the politician Masayoshi Shido - the father who abandoned him. While he yearns to be depended on and loved, he is cynical towards society to the point that he commits heinous crimes under Shido's orders, including committing murder, all in order to reveal Shido's megalomaniac true nature and cause both his downfall and the breakdown of society at large.Tristitia also ties to suicide and a lack of self-preservation; fittingly, Akechi is indifferent towards his fate after Shido's downfall: as long as his revenge is filled out, he does not care if he dies or is arrested for what he did. The second phase of his boss fight also has him berserking himself with rage with his Persona Loki, all in order to kill the Thieves.
In Dante's Inferno, Dante's will is steadily broken as he goes deeper in Hell and he is forced to face the many sins he committed in life. When a corrupted Beatrice calls him out on his misdeeds and betrayal of her trust, he falls to his knees and gives up on trying to redeem himself, deciding that he deserves to be trapped in Hell.
In Castlevania: Lords of Shadow, Gabriel crosses it right after a moment of utter triumph when he kicks Satan's ass when he sees that Marie is about to leave for the afterlife forever and that he cannot go with her. Realizing that the hope he believed in was hollow, Gabriel collapses and cries silently. Even centuries later, he has not recovered. As the immortal Dracula, all he does is hide in shadows while longing for a death that may never come.
Gerald Robotnik crosses this hard in the Last Story of Sonic Adventure 2 when he finds out about Maria's death.
Shadow The Hedgehog can fall into this during certain endings of his titular video game (i.e. discovering he is an android, believing he is an experiment gone wrong). However, other endings he'll completely avert it. Ironically, no matter what happens, he will always end the story by saying "This is who I am."
Besides the Last Story, which he says "Goodbye forever, Shadow the Hedgehog."
Sonic hits this in Sonic Lost World. Despite continuing on, he's incredibly affected as everyone seemingly drops like flies thanks to his bull-headed stunt that lead to the Deadly Six breaking free: his world has been sucked dry, Amy and Knuckles been hit with it, Tails captured by the villains and, for extra measure, he's forced to watch Eggman, Orbot and Cubot plunge into lava. He almost seems resigned when he sees Tails turned into a cyborg of sortsuntil he reveals he's still in control and not a robot. Eggman even returns, ready to reclaim his victory and, at the end, everyone's revived. For such a bright and colorful game, it's probably also one of the darkest.
King's Quest III explains that King Graham passed this when Rosella (his remaining child) was picked for the annual Human Sacrifice to the dragon plaguing Daventry. This is why it falls to "Gwydion" (aka the lost Prince Alexander) to rescue her and the kingdom.
(I... need to save... Tsubaki... but... I feel like I... like I don't... have... anything...)
Terumi feeds on others' despair to survive. He happens to be very good at causing this to extraordinary levels in almost everyone, and does exactly that, even if it's just for his own amusement. Every single ending, except his own, ultimately is a Heads I Win, Tails You Lose situation for the character. He uses his abilities to turn Noel, the girl who is loyal to him and falsely believes him to be a force of good, into a sobbing wreck of self-loathing and hatred.
His Motive Rant, which is delivered after said Heads I Win, Tails You Lose situation sums up that he thinks everyone who have crossed the Despair Event Horizon are the only honest people, everything else are lies, those who believe them are liars and he's willing to make his own brand of truth of "Everyone, cross the Despair Event Horizon" to become the accepted truth in the world.
At the beginning of Act IV in Diablo III, everyone in Bastion's Keep who isn't dead following Leah's death and Diablo's resurrection as the Prime Evil at the end of Act III comes damned close to crossing this. Haedrig, the blacksmith who's followed you faithfully throughout the game, comes probably the closest to it:
Haedrig: I thought I could make my wife's death mean something. It doesn't matter now, does it?
Player: It does matter. You are here now, and we might still turn the tide of this battle.
Haedrig: Right. I'm sure that's a comfort for Leah now that she's gone too. I've been a fool. You don't get to make things right. This world isn't made for redemption.
In addition, the angels of the High Heavens have also fallen to despair as a result of Rakanoth capturing Auriel, the Archangel of Hope. Only by killing Rakanoth and freeing her do you restore hope to both the angels and everyone else.
New material for the Diablo franchise reveals that Prince Aidan aka The Warrior from the first game went over this when he discovered that in killing Diablo, he killed his baby brother Albrecht, and saving Albrecht had been his driving goal. This may have driven him to jam Diablo's Soulstone into his forehead, an action that he probably would not have done otherwise!
Spec Ops: The Line - Walker goes through one at the end of the game when he realizes that he was hallucinating the villain, John Konrad, he slaughtered a U.S. Army regiment, and that he has doomed the entire city of Dubai to die slowly of dehydration. The rest of the squad goes through one much earlier after they unknowingly use whitephosphorus on a refugee camp.
In I Miss the Sunrise, a lacertian man named Chac seems to have passed this; he was the lone survivor of a ship that was wrecked by the Shine, and is wracked with survivor guilt. In addition, he lost both his wife and his legs. Turns out he hasn't quite passed the brink; though it takes a Herculean effort, his sidequest involves bringing him back from the event horizon.
In Odin Sphere, when Oswald learns that Gwendolyn handed over the ring Titrel, which he'd given to her as a wedding gift, to her father Odin, he takes it to mean that she does not love him and is trapped in their marriage. Given that Oswald had latched onto the possibility that Gwendolyn might love him as his last hope in the world, the thought that she has no feelings for him drives him so far into despair that he lays down his sword and lets the Halja drag him off to the Netherworld. Fortunately for Oswald, he's mistaken about Gwendolyn's feelings, and she promptly dons her battle tutu and storms the Netherworld to get him back.
This is what motivates Wolf to become a robber in PAYDAY: The Heist. According to his backstory, Wolf lost his job when the economic slump hit, his last client refused to pay him for his work, and he and his family became homeless. It was enough to send Wolf over the edge to the point where he suffered a mental breakdown and started to act out based on famous heist movies.
In Suikoden V, the Prince will hit this at the end of the game if you recruit less than 60 Stars of Destiny; He wanders off into the frozen mountains, utterly broken by his experiences.
In Mass Effect 3, Tali suffers one if you allow the geth to wipe out the quarians. She throws herself off a cliff, and there is absolutely no way to stop her.
Conversely, Legion suffers this if you choose to save the quarians at the expense of the geth, in fact, Legion decides to try and kill Shepard out of hopeless spite, but Tali shoves a knife in his back and saves Shepard while Legion dies.
Likewise, choosing to sabotage the genophage cure will cause Wrex to withdraw all krogan support from the fight against the Reapers and confront Shepard, which ends about as well as can be expected.
The krogan species as a whole is suffering from this - since their culture has a heavy focus on the idea that Might Makes Right, their defeat in the Krogan Rebellions due to the genophage has led them to view themselves as not even worthy of survival, and as a result they've degenerated into internecine warfare and mercenary work and don't put all that much effort into trying to cure the species-wide stillbirth plague.
Your clone, when Brooks abandons them in the Citadel DLC. If you take the Paragon option and try to save them, they flatly refuse to take your hand and simply...let go of the Normandy's ramp. Hundreds of feet above the Citadel streets.
Gavin Archer, if you took the Paragon choice at the end of the Overlord DLC and then fail to save Grissom Academy. You can also save Grissom Academy but lie about doing so.
God be with you, Commander Shepard. He was never with me. [gunshot]
In the backstory to Injustice: Gods Among Us, shown further in its prequel comic, Superman crosses it after he is tricked by the Joker into killing his wife Lois Lane and their unborn child thanks to Kryptonite-laced Fear Gas, and since Lois was hooked to a detonator to a nuclear bomb that went off when her heart stopped beating... Metropolis was destroyed on his account as well. Not only does Superman kill the Joker in revenge, but he is also so consumed by this and his unresolved guilt that he eventually becomes completely disillusioned with conventional superheroics and humanity's warlike and criminal natures and decides to turn the Justice League into a Repressive, but Efficient planetary government, while simultaneously shooting down any attempts to redeem him because he just can't accept that what happened wasn't his fault.
In The King of Fighters, Leona Heidern has spent years in the border of this after she killed her parents and townspeople under Goenitz's More Than Mind Control, with only a big dose of Trauma-Induced Amnesia and her life as a soldier holding the rests sanity. When she actually recovers her memories of this, she's almost fully crosses the line and then becomes Orochi Leona. Fortunately, her True Companions Ralf and Clark are there for her, so they manage to bring her back.
In Ib, Ib herself suffers this in two of the endings. If you do extremely poorly and then fail the Doll Room event, you get the "Welcome to the World of Guertena" or "A Painting's Demise" endings, in which Garry has been driven insane and Ib, though she tries, is unable to do anything about it. This leads to her collapsing in despair and never getting back up again, despite Mary's attempts to get her to wake up.
Dwayne crosses it in Grand Theft Auto IV if you decide to kill him in the mission "The Holland Play". It has been established that Niko is the only person he can rely on, and the closest thing he has to a friend. Realizing that Niko has decided to kill him pushes him past the event horizon. While most of the characters that you execute in the course of the game have to be chased down and cornered, Dwaynejust turns his back and waits for Niko to end him. Thankfully, there is a much better alternative.
Elizabeth crosses it in Bioshock Infinite in the Bad Future that results from her being recaptured by Songbird. In Comstock's attempts to get her to take up his mantle, she is tortured both physically and mentally, but holds on to hope that Booker will rescue her. However, he doesn't, and Elizabeth is convinced that he abandoned her; in reality he was stopped and probably killed by Songbird. With nothing left to hope for, she goes full on Woobie, Destroyer of Worlds and accepts the role of Comstock's successor, sending Columbia into war-preparations and subjecting any dissenters to a Mind Rape that transforms them into easily-manipulated lunatics. And when she realizes why Booker didn't save her, all her attempts to try and stop the monsters she's created fail. It's only through the intervention of the Luteces that can she find the will to prevent this future from ever occuring with the help of another version of Booker, and even when this plan comes to fruition, Elizabeth still can't see any hope for herself.)
If Ethan Mars of Heavy Rain fails to save his son Shaun at the end of the game, he will fall into this, ultimately leading to suicide. It doesn't help things that his other son, Jason, was killed near the beginning of the game.
In Skies of Arcadia this ends up happening to Ramirez after his mentor figure Galacian is killed. This leads him to bring down the Rains of Destruction and destroy the whole planet instead of conquering it, reasoning that Galacian was the only one who could save the world.
As a bonus Galacian is killed because one of his subordinates, Belliza, crosses her own DEH after realizing that he will always be a power-hungry tyrant and never reciprocate her feelings towards him.
The port city of Esparanza is an entire city of people who'd crossed the event horizon. The city was set up by Valua right next to the Dark Rift with plans to cross through it into the skies beyond, but after repeated attempts to cross ended in failure Valua stopped funding any future voyages and withdrew from the city, leaving an old dreary rustbucket of a city full of people who'd given up hope and are just waiting to die. Fortunately, Vyse's visit and eventual success in crossing the Dark Rift starts reigniting hope in the townspeople.
In NieR the eponymous protagonist gets hit with this HARD in the end of the first half of the game when he completely fails to save his sister (Replicant version) or daughter (Gestalt release) from the Shadowlord and is forced to sacrifice one of the only true friends he's ever had for a hollow victory against a Shade. This destroys the bubble of hopes and dreams he was trying to create for him and his surviving family member and turns him into an embittered and murderous Blood Knight, who revels in killing Shades as an outlet for his anger and despair as he tries to get back what he lost.
Pascal hits this very hard when, after he and A2 fend off invading machines to protect the child machines under Pascal's care, they discover that the children took their own lives. With every member of his village now dead, Pascal loses the will to live and asks A2 to end his suffering, either by killing him or erasing his memories.
Cody Travers from Final Fight and later the Street Fighter series. While it is not actually seen in the games, multiple games tell the story of his downfall, which occur after the ending of the original Final Fight. Cody and his friends go out to save his girlfriend from the Big Bad in Final Fight. On the way, he beats up a corrupt cop named Edi, who later arrests the hero for assault and battery. Next, his girlfriend dumps him, and leaves the country to study abroad. Afterwards, he is let out of jail and tries to get revenge by fighting criminals outside. He gets arrested again, and becomes addicted to fighting within prison. He then eventually breaks out, and joins the Street Fighting cast in their tournament(s). After all these events, he usually claims that he will never be the hero again, and often states that all he has left is fighting (which he often exclaims is pointless).
Schala Zeal of Chrono Trigger is left so broken in sadness and despair she willingly resigns to her fate of being trapped in the Dream Devourer. Not even her brother's will to save her lifts her spirits, infact his lust for power to that end only makes it worse.
In The Awakened Fate Ultimatum, if you choose to have Shin not try to save Eri after she is kidnapped by devils, he receives a package containing her severed head. This causes him to go full monster, killing everyone around him, destroying Celestia and then the devils that come to investigate the carnage. You are then dumped to the game's title screen.
In Pillars of Eternity, the entire country of Dyrwood is falling into despair thanks to the Hollowborn epidemic. It's pretty hard to keep your hopes up when your kids are born without souls after all.
Blank Dream has the final memory Mishiro needed to recover, which is her only friend and the only person she feels cares for her, Ayato, throwing her out of the way of a speeding car but getting hit himself, and dying right in front of her. The narration mentions her heart "dying in that instant", and that was the final straw that pushed her to throw herself into Mirror Lake. Worse, in two of the endings, Mishiro finds out that the person behind the wheel was Ayato's mother, and she was aiming for Mishiro.
Darkest Dungeon: Heroes afflicted by Hopelessness will cross this one and practically give up on leaving the dungeon alive. They will spend turns weeping and lamenting their own fates and those of their unfortunate partners, curse their luck, and even try to kill themselves, stressing everyone else out.
The Stormblood expansion is about the Player Character trying to pull the people of Ala Mhigo and Doma out of their own DEH and rise up against the Garlean Empire:
The Ala Mhigans tend to bounce up and down in this trope, but hit this pretty hard due to The Griffin/Ilberd's mad and suicidal plan to kill his people to create a brand new Primal.
Conrad, leader of the Ala Mhigo Resistance, hits this briefly after the Garleans raid Rhalgr's Reach, leading to the deaths of many people, including the man he wanted to succeed him. He's able to recover after awhile.
The Doman people have been in one due to a failed rebellion that cost them the life of their old leader (and thought to have cost them his son as well, but he's still alive, waiting for the right time to return) and Yotsuyu's petty retribution.
Yugiri hits this as she rescues a group of Domans, who are now scared shitless of being punished. The Player Character is able to shake her out of it.