"Jim": You can't kill me, either. Oma Desala: I can fight you. "Jim": Well, you can't win. Oma Desala: It won't matter. You won't be able to do anything but fight me back. "Jim": What're you gonna do? Oma Desala: Something I should've done a long time ago. (she attacks him)
In Inuyasha this is the true nature of the Shikon no Tama. On the verge of dying to a being similar to Naraku, a powerful miko used her powers to fuse her soul to the demon's, creating the jewel. The battle between miko and demon continue within the jewel, influenced by the wishes made upon it. Kagome, the heroine of the series, nearly ends up trapped in the same situation but manages to figure out the way to not only avoid that fate but also unmake the jewel and release the miko's soul.
The plot of Naruto commences with the Fourth using a variant of a sealing technique which summons a shinigami that devours both him and his opponent, damning them to do battle in its belly for all eternity. However, he used another technique with it that made the target (or maybe just half of it) instead become Sealed Evil in a Can. One wonders how the battle is going between The Third Hokage and Orochimaru's arms.
It's also mentioned that it might just be a legend, and it probably just kills you. It's not like any of the previous users or victims of the technique, being dead/eternally sealed in a demon's innards, are in any position to tell anyone. However, the phrase "summoning contract" implies terms are spelled out between the summoner and summoned. If the contract stipulates the summoner is going into the summoned's stomach, then it will happen if you want the rest of the contract carried out. We eventually find out from Kabuto that souls being sealed in such a fashion do have a different fate than those who die normally, as it prevents them from ascending to the normal afterlife on a different plane which in turn prevents them from being brought back by Impure World Resurrection.
The myth is eventually proved true when Orochimaru uses his body as a physical vessel for the Shinigami and slits his stomach open. This frees his arms and allows the Hokages to be summoned.
At the end of Getter Robo Armageddon, the original Getter heroes join an entire interdimensional legion of warriors dedicated to this. Though, considering that this iteration of the Getter team are pretty much a gaggle of Blood Knights, for them it's not so much Hades as Valhalla.
In New Getter Robo at the very end it's implied this version of Ryoma also joined them.
Xam'd: Lost Memories ends with Nakiami sealing herself in the Quickening Chamber for a thousand years to offset the darkness of the Hiroken Emperor.
In Code Geass, it is subverted. Lelouch seals himself and the Emporer in the Sword of Akasha. It works out pretty well, with Charles having just gained immortality... until Suzaku arrives. Then it culminates with Lelouch destroying his mother and father.
Of course, if things had gone according to plan, Lelouch presumably would have died of thirst, leaving the the immortal Emperor as plain old Sealed Evil in a Can.
In Dragon Ball Z, when Gotenks says he won't be able to defeat Buu (actually a trick to distract Buu and a failed attempt at making himself look cooler in front of Piccolo by being a showoff and defeating him at the last second), Piccolo destroys the only door leaving the Year Inside, Hour OutsidePocket Dimension they're all in. Subverted in that Buu escapes by yelling and creating a space rip, and afterwards so do Piccolo and Gotenks by doing the same.
Would have been subverted anyway, since Piccolo and Gotenks wouldn't take long to become exhausted and eventually die of dehydration. Piccolo happily points this out: "Our friend could still try to get rid of us if he wants, but then he'd be all alone..."
In the movie Dragon Ball Z: Wrath of the Dragon, the antagonist convinces the heroes to gather the dragon balls to wish that a special music box he brought with him will be opened. He states that it has a trapped hero inside who will foretell of the coming of a great evil. This is all true, but he fails to mention that the great evil was bound to this hero, and by releasing the hero the evil is also released.
Pharaoh Atem seals himself in the Millennium Puzzle with Zorc Necrophades. Played literally as the two of them spend the next 3,000 years jousting over the most complex RPG known to man: human history!
It is strongly implied that Yugi and Kaiba will keep on reincarnating and dueling against each other for all of eternity. Summed up in the Abridged Series:
Kaiba: Best. Destiny. Ever.
At the end of Puella Magi Madoka Magica, Madoka makes a wish to destroy every witch before they are born. In a strange sequence, she's told that she'll be locked in an endless battle with every possible witch. This is largely an Informed Ability, since we never actually see this happen - Madoka just causes magical girls to vanish before becoming witches. In fact the only witches we see her destroy are Walpurgis Night and her own witch. Notably, the manga adaptation seemed to change the line.
Claymore does it twice: once for Luciela, a former top warrior of the Organization-turned-Abyssal one, who is sealed away by her sister Raphaela (the two of them eventually become the Destroyer); the second time when dying Clare uses her last strength to seal both the Destroyer and Priscilla in a giant flesh cocoon, trapping them in a battle of willpower... for how long? No one is certain yet.
Done at the end of the Busou Renkin anime. Kazuki uses his Energy Release as a rocket to launch himself and Victor to the moon. Once there, they can't hurt anyone else but can never return until the rest of the alchemists get around to building a makeshift rocket.
Used in Tayutama. Mashiro seals herself along with the Tayutai to be there should they escape to recapture them all. The Tayutai aren't exactly evil (well, some are) but more playful and mischevious making them dangerous if let go.
In One Piece, there was once a pirate crew of giants that were basically unstoppable. Then, one day, a little girl asked their two leaders which one caught the biggest fish. The two warriors, Dorry and Broggy, began fighting over it, and as the rest of the crew split up are still dueling even to this day...for 100 years...
In Kenichi: The Mightiest Disciple YAMI (evil) do this to the good guys by luring their strongest (Hayato Fūrinji) into an equal fight. They have been fighting for days. It seems the winner will be the one to scrounge/hunt-down more nutritional food in between attacks, blocks and dodges.
Once, after Doctor Doom came within an inch of wiping out the Fantastic Four, Reed decided the only way to keep his family safe from Doom for good was to trap the two of them in an inescapable pocket dimension. Knowing the team would never allow him to do this, Reed tried to distract them and push them away emotionally, but they managed to stop him anyway (unwittingly letting Doom escape). The kicker? Doom was already in Hell when Reed resorted to this — with Doom's record, he didn't trust the devil to hold him.
and he was right! The comic shows Reed interrupting Doom in hell just as Doom was beginning to put a plan in motion.
Another FF storyline, when they faced Reed's time traveling evil grandson Hyperstorm, ended with Hyperstorm trapped in a pocket dimension with Galactus, constantly blasting him with his own limitless energy to keep him at bay, but with Galactus feeding off the blasts. At the time, the FF assumed the two might be stuck that way forever, with Ben commenting that it was an awful fate even for a crumb like Hyperstorm. However, Galactus has since reappeared, which implies that Hyperstorm either found a way to escape or else Galactus ate him.
In the Transformers Generation 1 comic books, this is how the planet Cybertron and the planet-eaterUnicron came to be — he and Primus were elder gods who battled across several planes, with the single-mindedly destructive Unicron winning the battle. As a last resort, Primus led Unicron from the astral plane into what is our dimension and directly into two metal asteroids, trapping both of them for eternity. While Primus turned his asteroid into the planet Cybertron, Unicron managed to turn his asteroid into a transforming planet-sized robot.
In Marvel Comics's what-if of their crossover Atlantis Attacks (in What If? v2 #25), Set the Serpent God succeeds in freeing himself from interdimensional exile and returns to Earth, killing most superhumans and converting the rest (as well as regular humanity) to serpent people. One by one the remaining heroes and villains fall, and all hope seems lost, until Quasar arrives on the scene, having just escaped Set's dimension (where he'd been lost since the Serpent God escaped). He's been granted the Captain Universe power, which combined with his Quantum Bands to make him Set's equal. He opens the recently deceased Doctor Strange's Amulet of Agamotto, using it to pull them both into a pocket dimension where the two of them will battle forever. Unfortunately, although Set has been removed from the equation, he's had time to reproduce...
Another example from Marvel - after Odin got Killed Off for Real in a battle with Asgard's arch-enemy Surtur, their battle continued on the doorstep of afterlife. Every day Surtur tries to reach a part of his spirit that will allow him to revive in the real world, and every day Odin kills him and then dies from terrible wounds, just for their injuries to disappear by the next day, so that their struggle may continue until the end of time. Odin doesn't mind though, seeing this fate as an atonement for his old misdeeds and mistakes.
An Elseworlds posits that Batman and the Joker will be fighting each other in one form or another for eternity. Batman Beyond: Return of the Joker also supports this notion, at least until Terry kills him.
This is how Palpatine is ultimately defeated in the Star Wars Expanded Universe: Empatojayos Brand sacrifices himself so he can carry Palpatine's spirit away from the universe and in the custody of every Jedi who has ever lived, never to be resurrected again. He doesn't have to undergo Fate Worse than Death himself, fortunately, but the effort of carrying away a spirit of pure evil does cause his own spirit a great deal of suffering.
An issue of the Dark Horse Godzilla comics featured Gekido-Jin, an invincible demon who is imprisoned in a statue by a warrior whose soul fights his soul for all eternity. When Godzilla decides to go sightseeing on the (still inhabited) island Gekido-Jin is imprisoned on, a minor character sacrifices himself to free the ancient warrior from his eternal battle and let Gekido-Jin live long enough to drive Godzilla off, at which point the man who sacrificed himself takes up the spirit-duel with the demon.
At the end of the Crimson comic, one half of the Big Bad Duumvirate, the king of dragons, who it's implied can never be truly killed (and thus it's implied could otherwise always recreate the dragon race in time), is dealt with this way when it and the current reincarnation of Saint George (yes, that one) are trapped in a Pocket Dimension to fight each other forever. Actually subverted in that God intervened, so that the dragon king would be Fighting a Shadow for eternity, while the real Saint George, after centuries of service culminating in a Heroic Sacrifice, is finally taken to his final reward.
While not exactly "sealed", it's revealed in the Sonic the Hedgehog series that the Xorda, aliens that eradicated most of the humans when Mobius was known as Earth, and Black Arms, aliens seeking to feast on Mobius' population, are locked in a vicious war with no end in sight over their claim of Mobius.
Used as a Back Story in The Sorcerer's Apprentice with both the Grimhold (with Morgana and Veronica) and with the Chinese vase in which Balthazar and Hovard get locked for 10 years.
It's implied that Balthazar and Horvath didn't fight in the vase, as Horvath claims he was extremely bored, reading the same (poorly-written) essays on Napoleon over and over. You don't really have time to do that if you're busy fighting. At least Balthazar had his Incantus.
One of the many alternate endings for Freddy vs. Jason had the pair locked in eternal battle in an arena in Hell.
In Twilight Watch, Anton, The Hero, has to go up against an uber-powerful vampire. He's given various spells by the forces of both light and darkness, as well as the Inquisition. The spell given by the latter will encase the person its directed against in a tiny coffin wherein they will be alive for all eternity. However, the same fate will occur for anyone who casts the spell. Luckily, Anton doesn't have to use it.
In The Book of the Dun Cow, Mundo Cani drives the Wyrm back into its prison inside the Earth where it's said that not only does he keep fighting to prevent Wyrm from invading the upper world, but that the souls of the brave go to join him in the battle when they die.
In Rick Riordan's The Red Pyramid, they deduce that this is what Bast was freed from, meaning that what she was fighting now has a chance. (She had to be; she was weakening, and needed to recover her strength.)
In The Sorceress, Aoife willingly triggers this to stop Coatlicue from rampaging across the myriad worlds.
In Riddle of the Seven Realms by Lyndon Hardy, Astron can only stop Palodad from destroying the multiverse by pushing the Big Bad into the flame that's tearing a hole in the daemonrealm, then jumping through himself and pulling the source of flame in after him. Subverts this trope, as Nimbia is able to create a new pocket-world within the Void to encase Astron, then retrieve him.
This is how the Penhaligon trilogy ends: Fain Flinn will fight Teryl Auroch forever to keep him from destroying Mystara.
This is how Superman wins against Saturn, The Devil's Agent on Earth in the novel Miracle Monday: by threatening to spend every second of the rest of his life fighting him and undoing all his evil deeds. (Actually, the book makes clear that Saturn could've won easily and destroyed the Earth if he wanted- but neither of those were his goals; he wanted to break Superman's spirit and end the hope he inspires in humanity. But just the realization that Superman would never give up effectively caused his defeat due to the laws that govern demons.)
At the end of A Night in the Lonesome October, Lawrence Talbot pounces on the evil Vicar and they both go tumbling through the otherworldly gateway, which seals behind them. This not only traps the Vicar where he can no longer open the way for an Eldritch Abomination invasion, but also confines Talbot somewhere where he can no longer harm people as a werewolf.
A variant in the Dr. Seuss story The Zax: A North-Going Zax and a South-Going Zax bump into each other, and each is too stubborn to go around the other, since this would require them to move slightly to either the east or west. After a brief argument the two are simply stuck glaring at each other, as years pass and other people build highways around them.
In The Golden Cat - the sequel to The Wild Road - the mentor character and the villain from the first novel are locked into this.
This is how the Dreadking is defeated in the final volume of The Riftwar Cycle. The Dreadking is an Eldritch Abomination, personification of the Ultimate Evil, that is too powerful for any ordinary magic to contain and will ultimately consume the entire universe, past, present, and future, if freed. Against it are set Pug and Tomas, a human who possesses the power of a Valheru. To fully tap into that power, Tomas has to let himself be subsumed by the Valheru personality, an Above Good and Evil demigod warlord, but that's the only way he can be powerful enough to grab the Dreadking's undivided attention. So Tomas unleashes his Superpowered Evil Side, goes in to fight the Dreadking- and then Pug uses all the power he can to trap them both eternally in that moment, preventing either from becoming a threat to the world and warding away the Dread from the living world for good- at the cost of his own life and much of the surrounding geography.
An interesting variant in Wheel of Time: evil can sometimes be fought not by good but by another kind of incompatible evil. First, Rand is wounded by the ruby dagger, a weapon infused by the evil of the dread city Shadar Logoth, which died centuries before because of his own inhabitants' growingly treacherous, xenophobic and egoistical behavior. He barely survives it, and despite magical medical attention the wound continues to slowly fester and is expected to eventually kill him. Later, he is stabbed at the exact same place by Ishamael, the main champion of the Dark One. Instead of making things worse, he observes that the Dark One's evil and Shadar Logoth's evil are locked in fight against each other and that the progression of the disease is stopped. He learns from that and uses it as a model/proof of concept for a later magical work, where he removes the taint the Dark One inflicted on Saidin using the dark energies of Shadar Logoth.
Star Trek: The Original Series: In the episode "The Alternative Factor", Sane!Lazarus asks Captain Kirk to imprison him and his evil, normal matter counterpart in a time corridor, where both will remain fighting in a form of living death for all eternity. Since refusing to do this would put the universe at risk, Kirk reluctantly agrees to carry out this plan. Fridge Logic kicks in because, unlike almost every other example of this trope, Lazarus is not a god-like being who can only be opposed by his opposite half, he's just an ordinary man with slightly better advanced technology. There's nothing to stop them from simply shooting the evil Lazarus, or at least handcuffing him to a pipe.
A nod to this trope was seen in the Doctor Who episode "Last of the Time Lords". After the decisive defeat of The Master, the Doctor was not willing to execute the only other member of his species in existence. So he volunteered to live up to this trope, keeping the Master confined in the TARDIS for all eternity or he reformed. The Master prevented this by committing suicide (or at least refusing to come back from the dead).
In the season 5 finale of Supernatural Sam combines this with Sealed Evil in a Can by inviting Lucifer to possess him, then trying to keep control of his body long enough to jump into the Can they have no chance of getting Lucifer into otherwise. Sam does end up jumping into the cage, with Michael (In Adam) falling in with him.
In Magic: The Gathering's "Innistrad" pack, the whole plot is driven by the fact the archangel Avacyn got herself trapped in the Helvault alongside her demon nemesis Griselbrand.
In the Eberron setting, the Church of the Silver Flame was founded around a Lawful Good energy phenomenon supposedly created when Tira Miron and a couatl merged together to trap an archdemon inside of it. Tira's voice is often heard from the flame, though there are heretical rumors that the couatl and demon sometimes speak from it also.
This is the origin of the Calim desert in the Forgotten Realms Dungeons & Dragons setting, in the form of a never-ending duel between a djinn and efreet lord.
Sisyphus shows up in the Fantasy Kitchen Sink that is Scion, and his specific case is subverted - when the Titans broke out, a rock took out the top of the mountain, letting him finally finish the job. As a result, he is now Nigh Invulnerable, possesses great power... and has no clue which side to sign on with. (If the Titans win, the world is screwed; on the other hand, three thousand years of pushing a boulder is a long time...)
In prior editions of the game, to stop the Thirteenth Black Crusade, Eldrad Ultharan had to trap his soul in a battle with the Chaos spirit powering Abbadon's Planet Killer. There was significant disagreement about whether he was dead or not, but given that it's now been retconned there's not much to argue about anymore.
Similarly, an Ork vessel once crashed on a planet in the Eye of Terror. Everyday, umpteen million Orks awaken and wage unending battle against countless hordes of demons until they die horribly, only to reawaken the next day. The Demon Lord in charge of the planet uses this to train and harden his army, except they keep getting killed and weakened; the Imperium thinks of this as a Fate Worse than Death; the Orks consider this paradise.
On the Warhammer fantasy side, Caledor Dragontamer and his disciples have been trapped in the Vortex on the Isle of the Dead, where they have been stuck for at least 3000 years, ceaselessly performing a magic ritual designed to pull the excess magic out of the world and prevent (or at least massively delay) the victory of the Chaos Gods. In an interesting variation, the Chaos Gods are not on, or even touching the world near the Isle of the Dead (the closest being the overlap between the "real" world and the Realm of Chaos at the north and south poles), nor is any champion of Chaos present. It just happened to be where Caledor chose to set up his world-spanning spell.
Kingdom Hearts: Birth by Sleep reveals in the secret ending that Terra is doing this with Master Xehanort as of the end of the story, both of them fighting for control of Terra's heart. Of course, a year or so later Terranort splits in two, his heart becoming False Ansem and his body becoming Xemnas; what they've been doing since then is anyone's guess.
Dream Drop Distance indicates that both Ansem and Xemnas know about Master Xehanort and their defeat recreated him; what has become of Terra in the decade since then is unknown.
In Pokémon, Wobbuffet was given the ability Shadow Tag in Generation III. This prevents the opponent from switching out. If two Wobbuffet battle each other, then the battle would last until they run out of attacks and are forced to use Struggle. However, a hold item called Leftovers would heal the Wobbuffet every turn if the Wobbuffet was holding it, and since it ended up healing more damage than Struggle was dealing, having two Wobbuffet in battle both holding the Leftovers item would result in a never-ending battle, causing Wobbuffet to become the first non-legendary Pokémon to be banned from official tournaments. This was fixed in Generation IV, where Shadow Tag will fail to work if the opponent also has Shadow Tag, and Struggle doing 1/4 of maximum HP in damage with recoil.
Blizzard Entertainment likes this trope.
It begins in Diablo. This is what the protagonist attempts after slaying Diablo's current avatar. Needless to say, it didn't work.
In Diablo II: In the backstory (actually already in the first game's manual), Tal Rasha used his own body as an extension of a soulstone to imprison Baal. He is possessed, and has to be tied up and magically bound in a tomb, his spirit fighting Baal's for eternity. Or until Marius came along in the second game and tugged on the ringpull in an attempt to rescue Tal Rasha.
In World of Warcraft, Arellas Fireleaf, a minor character of whom we only know what is written on a statue, is said to be "Locked in eternal combat with the Necromancer Diesalven".
A far more important example, if it is one: At the end of the Icecrown Citadel raid, Bolvar Fordragon dons the helmet of the Lich King to contain its power. This may mean that he's in there together with the original Lich King, Ner'Zhul, or that Ner'Zhul was destroyed or weakened to insignificance too and Bolvar is only keeping the armies of the Scourge at bay through his new control over them.
The cancelled Ultima X: Odyssey: When The Avatar sealed himself and his Enemy Without, The Guardian, behind a wall of life to destroy them both with an armageddon spell, The Guardian managed to merge with him. They then struggle for dominance with The Power of Friendship backing them up.
Blue's ending in SaGa Frontier appears to be him locked in an eternal battle with Hell's Lord after the portal to Hell is sealead away and the battle cuts away to a "The End" screen abruptly. Word of God says he teleported out, but that's not nearly as cool...
Quest for Glory IV has this with Erana and Avoozl. Ultimately, the hero's reason for being summoned is to finish summoning Avoozl, breaking the stalemate in favor of the Eldritch Abomination, only to free Erana who, now unbound, can banish Avoozl properly.
Freedom Force vs. The Third Reich: Manbot and the Wraiths of Chaos.
Richter Belmont: Count Dracula rises but once every century, and my role is over. If I can resurrect him, then the battle will last for eternity!
The motivation seems slightly different; as the above quote shows, Richter doesn't want to do this to trap Dracula (vs. letting him free); he wants to do this to continue the fight (vs. the fight being over).
Towards the end of Breath of Fire II, it turns out that the sleeping, giant dragon near the village of Gate is actually a Dragon Clan member (Ryu's mother, in fact) who turned herself into that form to keep the game's Hell Gate closed and prevent the release of Death Evan. Slightly subverted in that you must relieve her of this task by going through the Hell Gate and defeating the Big Bad. If you choose not to, Death Evan eventually overwhelms her defenses and bursts to the surface, resulting in a Nonstandard Game Over.
Megaman Battle Network 6 has an interesting twist: There were two great beasts ravaging the world, so they enticed the beasts into fighting each other and then blocked off the battleground as best they could so noone would interfere or get caught in the crossfire (obviously that didn't work).
In The Legend of Zelda: Skyward Sword, it's revealed that the Master Sword, the iconic weapon of the series, is this. The sword's spirit Fi takes it upon herself to seal Demise's soul within herself. Demise manages to unleash one last parting shot by unleashing a curse on Link and Zelda that would doom them to face an incarnation of his hatred whenever they are reincarnated. That incarnation turns out to be the series Big Bad Ganon.
For the very last fight of Duel Savior Destiny Taiga decides to invoke this one against God himself and proceeds to travel to a different dimension and do just that. Due to previously mentioned time distortions between realms, he goes on to win the fight several million times without being able to actually kill his opponent, which he knew would probably happen. Still, he manages to put a seal over God eventually, letting him return home.
The ending to Doom 64 has shades of this; after once again entering Hell to stop the Demons from attacking humanity, the Doom Marine decides to stay there for good, in order to prevent them from rising up yet again.
Calypso's ending in Twisted Metal 4has him demand the return of the ring that holds the power to grant wishes, and implicitly, the competition itself. When Sweet Tooth refuses, Calypso attacks him and wrenches the ring off his finger - whereupon the souls within it explode out. Later, a young boy finds the ring, and sees Calypso and Sweet Tooth still battling each other within it...
In The Elder Scrolls V: Skyrim, after inadvertently waking the Dragon Priest, Morokei, in the depths of Labyrinthian, Savos Aren contained him by enthralling his dead colleague's spirits in an eternal magical duel, preventing Morokei from being unleashed upon an unsuspecting Skyrim. He feels guilty about it to this day; the nature of the spell meant he couldn't sacrifice himself in their place.
Although it's unknown whether or not they actually fought, this is what happened with Tikal and Chaos in Sonic Adventure, as Tikal sealed both of them within the Master Emerald to prevent Chaos from completely destroying the world many years prior. Eggman shattering the Emerald and freeing both of them kicks off the main plot.
In the world of Tales From My D&D Campaign, the Big Good Ioun discovered the Source of all magic aeons ago. Unfortunately, the Big Bad Vecna learned of the Source almost immediately thereafter, and ever since they have been locked in a duel for control of the Source. Should Vecna win, he will be able to destroy all other gods, strip all magic from the world, and rule unopposed and unopposable. To this point, Ioun holds Vecna at bay, but at any moment the end could come.
From the same series, a sort-of-reformed Venom ends up jumping into a portal after his Evil Counterpart, Carnage, and since the portal closes after that, it's implied that this will be their fate. A sort-of Sequel Series saw them on another planet/dimension, though.
Jackie Chan Adventures ends with this, as the fifth season's Big Bad Drago (using the powers of all eight demon sorcerers) and his father Shendu (the Big Bad of the entire series, using all twelve of his talismans) are sealed within another realm to duel for all eternity. Neither of them takes their fate very seriously, and they spend their time bickering: Shendu chastising Drago for being an impudent child playing with his father's world (yes, Shendu still thinks Earth is his for the taking), while Drago whines about Shendu never being there for him because he's always busy fighting wizards.
One episode of The Fairly OddParents had a variation on this. Timmy wished Vicky wasn't his babysitter, and she became mayor of the city instead. Subsequent attempts to remove Vicky from power through wishing instead resulted in Vicky gaining even more power, up to the point where she became a galactic conqueror. In the end, Timmy realized it was up to him to prevent any of these from happening, and wished her to be his babysitter once again.
As for a closer example, there is an episode where Timmy wishes both of his parents were the best surfer. Due to the impossibility of two people being best at the same thing at the same time, they get locked into an eternal surfing competition known as Wish Limbo...limbo...limbo...limbo... However, it's broken when they find something that's more important to them than wishing, Timmy, who is attacked by the Kraken he wished up to try and scare them out of Wish Limbo...limbo...limbo...limbo...
Adventure Time has Goliad, Princess Bubblegum's genetically engineered heir who'd gone mad with power and tried to take over the Candy Kingdom with her mind control powers, ultimately defeated by Stormo, another Candy Sphinx created using Finn's DNA, sacrificing himself by locking her in a psychic duel with himself. Nobody thinks to just kill or knock out Goliad now that she's incapacitated.
In the original storyboards for "The Lich", Billy tells Finn about the Crystal Citadel, a prison for the multiverse's most dangerous criminals. He also says that Finn's birth father was unjustly imprisoned there and has been battling the criminals held there ever since.
One episode of Mighty Max contains this: An immortal, invincible caveman (the good guy) and an immortal, invincible sabre-toothed tiger (the villain). Originally sealed together in a cave, construction excavations accidentally freed both. Ultimately, since neither one could be destroyed, the caveman dragged the sabre-toothed tiger down to the bottom of a tarpit, where the two of them continued to battle, possibly until the end of time.
In the season 2 finale of Wakfu, this seems to be the fate of Goultard the Barbarian and Rushu, king of the Shushus, after the former drag his opponent back to his demonic realm. Indeed, the "Where Are They Now?" epilogue shows them still facing off through Igor the Shushu mirror.
In the backstory for The Legend of Korra, the Ultimate Evil Vaatu was kept contained by a centuries-long duel with the light spirit Raava. This fell apart when Vaatu tricked Avatar Wan into thinking that Raava was a bully that needed to be taken down a peg.