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Long ago, An Ancient People faced a terrible evil. Using various methods, they bound the evil into a prison from which they thought it could never escape.
Sealed Evil In A Can, as the title suggests, is a way to introduce a villain suddenly, especially one that is legendary and powerful. It also explains why the villain hasn't done anything up to that point: It just now escaped.
Sometimes, the Big Bad's plan is to unseal the can of some ancient God of Evil; hoping they'll get some of that great power as a reward. If they succeed, it almost always turns out that the Sealed Evil was manipulating them into freeing them, making the Sealed Evil the trueBig Bad. Sealed Evil™ almost never rewards those who release it. It may act like the malevolent flavor of a Jackass Genie, twisting their releasers' wishes back on them, or it may simply decide that You Have Outlived Your Usefulness. Remember: Evil Is Not a Toy.
The hero occasionally breaks the seal himself due to lack of information or active manipulation by the villain.
Oddly enough, no matter what the cause of the seal being broken, it seems to always happen 1,000 years after the evil was first sealed. Yet nobody ever seems Genre Savvy enough to know that the Big Bad is about to reemerge in that time frame.
The primary question is usually "Why did they just seal it, as opposed to KILL it?" Well, the answer tends to vary; but usually it's a variant of The Punishment (for both the sealed and everyone else), and/or As Long as There Is Evil. Maybe it can't die (or more commonly, the conditions weren't right to deal the final blow), so sealing is the next best thing. Another common excuse is the Balance Between Good and Evil; if they'd killed the ancient evil, then a bigger, badder evil would've taken its place, thus sealing it away neutralizes the threat but also keeps it in the world and maintains the balance. For some reason, only on a very few occasions is the evil sealed simply because the goods didn't have the power at the time to kill it outright.
Expect the mere release of the SEIAC to cause a World-Wrecking Wave, Walking Wasteland, and sundry other disasters. That said, being cooped up for centuries is likely to have weakened the SEIAC, meaning it needs some time to recover its lost strength. This could mean that the hero(es) has a chance to seal it back up, or even destroy it once and for all.
Since this trope can be traced back to Greek mythology, it's Older Than Feudalism. Notice that it is exactly the same, without the mystical magical mumbo-jumbo, as any plot where a horrible criminal escapes from or is released from prison.
This is an extremely common plot and backstory in video games because it's such a simple backstory: the Big Bad, put away by some ancient hero, has escaped. You (the player) must put him back in.
For a much older example, consider the Bottle of Suleiman, which upon being sacrificed has a 50/50 chance of either releasing a fairly powerful djinn that joins your forces or else simply blowing up in your face.
And then there's the Tomb of Urami, who nets you a big demon at the price of sacrificing all of your lands.
Yawgmoth, the guy who makes planeswalkers look like muggles and Dominaria's version of the Devil, was sealed in a whole other plane back when he was a mortal. He wanted back in and spent thousands of years in a battle of wits with Anti-Hero Urza with his freedom as the stakes.
In the Zendikar expansion a group of Eldritch Abominations called the Eldrazi were got sealed away by Sorin Markov and two other guys, turning all of Zendikar into a prison for them. The final set in the Zendikar block, Rise of the Eldrazi, sees them loosed on the Multiverse.
A plot-relevant example can be found in the Innistrad expansion: The Helvault was created by the archangel Avacyn to seal away demons that she couldn't outright defeat. The plot of the expansion is set in motion when, by twist of fate, Avacyn herself becomes sealed inside the Helvault.
Certain cards allow players to set up Sealed Evil in a Can situations. Cards like Oblivion Ring and Journey to Nowhere let a player seal an opponent's creature in a can but any card that can remove the enchantment frees them. Summoner's Egg goes the other direction, letting a player seal a creature from their hand and bringing it to the battlefield for free if the Egg gets destroyed.
Yu-Gi-Oh!! has several monsters like this, the mo st memorable being Exodia (who is split up into five pieces and allows you to win the game should you have all five in your hand; in the anime, it did take a huge ritual to unseal it) and all versions of the three God Cards (Egyptian, Sacred Beasts, and "Wicked Gods", all of which need three sacrifices to bring out and have devastating power, and two of which have fusions of themselves that are even more powerful). In the anime, they are so powerful that most of them are sealed up themselves, and it's considered madness for anyone to try and duel with them in their deck.
In "The Garden" segment of Garfield: His 9 Lives, Garfield (who is a kitten here) and Cloey (his owner in this life) are given a magical garden by Uncle Tod when he joins the circus, under the condition that they never open the crystal box on the checkered toadstool. The trope is subverted here because, out of loyalty to Uncle Tod, they decide not to open it.
In the comic strip Wormy, the title dragon owned a collection of magical orbs containing vicious demons, which a human wizard attempted to steal. This being a humor comic, Wormy used the orbs as snooker balls.
Mare of Steel: At the end of the story, Brainiac is petrified by the Elements of Harmony, and Rainbow Dash/Supermare puts him in storage in the Fortress of Solitude for safe keeping.
The Princess of Themyscira: Ares' master plan is to unleash the army of demons created by the Olympians' war with the Titans and then locked away in Tartarus, in order to start a Forever War. While he briefly succeeds in releasing them, Twilight (empowered by the Helmut of Nabu) sends them back, and Ares himself ends up sealed with them.
Titan from My Little Unicorn, at least till he breaks out from his dimensional prison.
In addition to Discord (who is imprisoned as in canon at the beginning of Reharmonized Ponies), the Pony POV Series has several others. One is Morning Star, Celestia's Fallen Alicorn brother, who was imprisoned for trying to take over the universe after deciding he was better than the Elders and being as evil as Discord if not worse. He was released by Havoc to fight in the Alicorn/Draconequi War, where he was either killed (though considering Evil still exists, he wasn't killed as thoroughly as it's possible to kill a God) or imprisoned in Hell (Havoc's domain) after the war as part of the peace treaty between the two according to Word of God. Another recurring one is Grogar, a G1 My Little Pony villain and (in this series) DarkseidExpy who is sealed away in another dimension. It's stated he returns every five hundred years unless stopped and has to be resealed. In his case it may be justified because, in Dark World, he's been shown to be powerful enough to go toe to toe with Discord and have a chance of winning, as well being extremely Dangerously Genre Savvy. Discord shows a great deal of anger that Grogar doesn't seem all that mad about this, in direct contrast to Discord himself, who can't stand his can.
There's also the Shadow of Chernobull, an entity created by Pandora and then locked in her Box, until a Hooviet experiment accidentally freed it, leading it to feeding on the Hooviets' existence and desires for a Super Soldier, becoming General-Admiral Makarov (Big Bad of Shining Armor's side-story).
Clover's story in the 7 Dreams/Nightmares collection reveals the existence of a group of Eldritch Abominations who fled into the POV reality when their own ended and tried to absorb all things into their Hive Mind in order to torment them for eternity. This offended both sets of the Elders (for different reasons), leading to them and their respective children working together to defeat these beings and seal them beyond the Gates of Tartarus. Notably, unlike most sealed evils, being sealed is having a negative effect on them; according to Rota Fortuna, unless they get loose and begin absorbing beings again, they'll devolve into little more than shadows of themselves (Luna states that this fate has actually already befallen many of them).
The Powers Of Harmony has Nightmare Moon's corrupted army, which was sealed in Tartarus. Keeping them sealed, and then preventing Cetus from releasing them, is a driving part of the plot.
Discord is also mentioned several times. Turns out, Harmony is trying to arrange his release simultaneously to her own, in order to purify them both and restore their balance.
In the Ah! My Goddess fic Ah Archfall, Jago's alter ego Papa Jupiter is one of these to the point of him taking over and killing Lind and their son being the one thing he fears the most. Unfortunately, he is also Jago's One-Winged Angel and so he doesn't stay quite as sealed as Jago would like. It is also implied that Gandamak was one of these before he was released by a traitor and it is also implied that the mysterious Titans are or were examples of this.
Herobrine is this in Yognapped. Eons ago, Notch sealed him away beneath the bedrock, where he waited until Peva set him free. He then goes on a rampage to destroy his brother's land. It's revealed just before his defeat that he isn't really evil, and that Notch is a manipulative Jerkass.
Demon Of Wind: This gets discussed in the story between characters as to why so many great evils are locked away instead of outrightly destroyed. It is also played with in the case of Rai who, though being locked away for being part of the bad guys, was at worst a Punch Clock Villain, albiet an alarmingly effective one.
In Fuck The Jesus Beam, BaROCK Dominatus Caesar Inferno Vespucci Giovanni Goku Super Sexy Awesome Absolute GOD O.B.A.M.A., officially known as the greatest evil the world has ever known, is a Kenyan terrorist who was president for 14,657 consecutive terms and started 95 world wars who has been sealed away for 100 million years. Niamy releases him to deal with the Bombignats and Fetii, only for him to take control of them.
The Touhou fanfiction Imperfect Metamorphosis has both Rin Satsuki and the Shadow Youkai, who were sealed in the different ways and for different reasons, although while they're both thought of as evil only one of them actually is. The characters involved in the sealings also spend a lot of time justifying them, which ultimately boils down to the two being too damn tough to kill and requiring a different method.
In Retro Chill, Retro and Bob end up stuck in a box after his last bout, and they are accidentally freed by Rupert Chill and Earl.
Time Lords and Terror: The plot is driven by Hydia's attempts to unleash the S'muz, in her misguided belief that it will aid her in her plans. She manages to free it, but after it kills her, the Mane Six proceed to use the Elements of Harmony to destroy it.
The Queen is trapped in Jade's mind, with the Tiger Talisman's power taking the form of two tigers that keep her from making any direct moves to control Jade's body. Eventually, her corruption reaches the point that the talisman separates them, freeing her from Jade. However, after she's defeated in the Final Battle, she's reabsorbed by Jade and trapped again.
During the Final Battle, Kyosuke ends up sucked inside the Urn of Wei Cheing by Uncle and Tohru.
Eugenesis has Unicron still sealed inside the Matrix, which proves to have been a bad decision all round. Not only does it weaken Rodimus just to hold the Matrix, it means he can't even use it for fear of letting Unicron lose. As the follow-up stories show, Unicron's mere presence proves corruptive, subverting the mind of an Autobot who came too close to an invalid Rodimus, and turning the Matrix Flame black.
In A New Chance For Adventure the attack on the Oldale Ruins by Team Magma releases an evil spirit that was responsible for the imprisonment of the Father Latios in the Soul Dew. And judging from Skailyn's anger, was responsible for a lot more evil in the ancient past.
Doctor Who fanfic The Door of the Demon has a door on the Green Moon behind which a demon is imprisoned. The Eleventh Doctor barely stops a criminal breaking them out.
Jafar becomes one at the end of Disney's Aladdin after he wishes to become a Genie and he gets trapped in a dark lamp. He gets freed in Aladdin: The Return of Jafar by a thief, and goes back to Agrabah to get his revenge on the heroes.
Mandrake near the end of Epic. And in a tree, technically.
The Titans in Disney's Hercules film. Zeus imprisons them all beneath the ocean with lightning bolts, but when the planets align, Hades is able to release them. Hercules uses the Tornado Titan to suck the other Titans in, then throws them all into space where they explode.
Uwe Boll's Alone in the Dark (2005) has this, though watching the movie it can be hard to tell exactly what it is. The movie talks about a dark world that mirrors our light world, but at other times, it suggests that the Abcane tribe that discovered them ARE the evil in the can. But then, it's Uwe Boll, so...
The title monster of The Beast from 20,000 Fathoms, while more predatory and aggressive than actually evil, is certainly an example of this. The rhedosaurus was frozen in Arctic ice since the early Cretaceous, and is thawed out by nuclear testing. Somehow, it is still alive.
Beetlejuice. The title ghostly con artist is trapped in the afterlife. He can be temporarily brought back to the real world (and returned) by saying his name three times, but can only be truly free if he marries a human female.
The original has the heroes create a sealed evil in a can out of the title menace by freezing the monster and transporting it to the north pole. And now we have Global Warming...
The sequel Beware the Blob: a oil worker brings home a frozen chunk of the Blob, sealed in a thermos. But not sealed too tightly or else we wouldn't have a movie.
The 1988 remake ends with the apocalyptic town preacher who was scarred by the bigger Blob and apparently driven quite insane in the interim revealing that he has kept a small part of the Blob in a glass container, and will unleash it when God will tell him it's time.
Demolition Man Phoenix was thawed out of cryo-prison to take out people who dont like Cocteau. But he has his own agenda and plans to release every other criminals in the prison to create his own army.
In the first movie, the Ghostbusters seal all of their captured ghosts in a large containment unit that can only stay locked through the help of a power grid. When they capture an individual ghost, they suck it into a smaller container. And it then gets unlocked.
The Big Bad of the sequel is a 16th-century despot whose spirit inhabits his portrait.
In Gremlins 2: The New Batch, Billy traps the Electric Gremlin in the Clamp Corp. phone system, where the villain suffers the torture of being on-hold.
The comic books as well as the movie adaption of Hellboy feature several such sealed-away monstrosities. In the movie, one demon was sealed in a consecrated urn, and the Ogdru Jahad, the Lovecraftian chaos gods of the Hellboy universe, lie imprisoned and waiting in the Void beyond the stars.
In Clive Barker's Hellraiser, the Cenobites will not come after you unless you solve the puzzle of the Lament Configuration.
In Highlander III: The Sorcerer, the villain Kane and the other two evil immortals are trapped in a cave for centuries, only being released in the present to attack Connor once again.
In Jumanji, the eponymous board game is buried underground for decades until it is unearthed by a construction team and found by Alan. "What if someone digs it up?" "Then may God have mercy on his soul."
The Keep: An ancient monster has been trapped in a castle for centuries, until some Nazis awaken it.
Played with and subverted in Kung Fu Hustle. The Beast is locked up in an insane asylum — but only because he was bored from the lack of worthy opponents to duel and so checked himself in. When Sing is snuck in by the Axe Gang to break out the Beast for recruitment and picks his cell door open, the audience sees the Beast as a bald old man with a pair of spectacles in wifebeaters and boxers sitting on the john reading a newspaper. Which makes the Beast Sealed Evil On the Can.
The Last Lovecraft: Relic of Cthulhu has the titular Cthulhu himself sealed (by choice) somewhere under the sea. The last remaining Lovecraft must guard the relic of Cthulhu to prevent it's awakening.
Living Hell, a recent horror movie has sublevel 4, vault 12, on a military base, which the protagonist doesn't want disturbed. The military had completely missed the hidden door in the vault 'til the protagonist waltzed in and told them there was something there... and we all know what to do with hidden doors, right? So, nice job there, hero.
In The Mighty Boosh Live, the Hitcher and his minions are introduced into the show when a box is opened. The Hitcher was apparently sealed in the box for 200 years, for crimes against humanity. Including, but limited to raping a dolphin and driving contaminated swans to fights.
In the Mother of Tears, Mater Lachrymarum's powers are restored by the unearthing of a lost talisman, making it more a case of "Sealed Evil Artifact In A Can." Lachrymarum herself was apparently in some sort of self-imposed hibernation prior to this.
2001's sequel, The Mummy Returns, opens up another can of evil in the form of The Rock's CGI-animated appearance as The Scorpion King — which in turn opened the door for one of those prequels mentioned in the main text.
In Mighty Morphin' Power Rangers: The Movie, the shapeshifting Ivan Ooze had been trapped underground for 6,000 years until his containment chamber was accidentally unearthed by a construction crew and later opened by Lord Zedd.
John Carpenter's Prince of Darkness had sealed evil in a can — The Anti-Christ, in fact, sealed in liquid form inside a giant glass and metal container in the basement of a church. In one of the sillier examples of this trope, the canister is designed in such a way that it can only be opened from the inside and the sole reason the son of Satan doesn't try to escape before the events of the film is that he was asleep.
And then there's The Thing (1982), sealed in a block of ice until some ill-advised Norwegians dig it out.
In Prometheus, the jars inside the structure, which can cause mutation or destruction to anything and everything that comes in contact with it. Later on, the expedition team discovers that the Engineer ship has a cargo hold full of these jars, along with a dormant Engineer who put himself in hypersleep, and intends to use the craft to destroy Earth via releasing the jars.
Quatermass And The Pit has an ancient alien evil buried under London for millions of years, finally unearthed by building works.
Queen of the Damned: Akasha's bloodthirsty rule over humanity was stopped when she and her husband turned into statues and slept for centuries. Lestat briefly awakens her shortly after being turned and again draws her attention when he starts to live in the limelight as a rock star.
Rare Exports: A Christmas Tale: In prehistoric times the Sami-folk of Lapland, tired of the tyranny of Santa Claus, lured him on weak ice, and he sank to the bottom of the lake, which subsequently froze through. He was cut out and covered with tons of sawdust to keep him from melting, and then layers upon layers of dirt and stones over the centuries, until the Korvatunturi mountain on the border of modern Finland and Russia was formed. And this Christmas an obsessed American millionaire decides to dig him out. He is not happy.
In the movie The Ruins, as well as in the book, the protagonists are attacked by a sentient, carnivorous, and parasitic vine living on an ancient pyramid (or in a mine shaft, in the book). The vine is kept on the pyramid by a ring of deforested and salted land carved out of the jungle around it, and is effectively sealed up by a very stringent quarantine set up by the villagers living nearby. In the end of The Movie, one woman manages to escape, but it is hinted that the vine is living inside of her, and in one alternate ending, it is directly shown that she has taken the vine with her back to civilization, unsealing the evil and letting it go free.
The Syfy Channel Original Movie Scarecrow has the title creature, who was Buried Alive underneath a farm until accidentally being released. Justified, as the creature is literally impossible to kill and will simply regenerate from anything done to it, including being shredded to bits, so sealing it away is the only way to stop it. At the end, the main character traps it in a boat and sinks it, imprisoning it at the bottom of a river.
Played with in the film adaptation of The Shadow. A museum receives a silver sarcophagus from a mysterious source, with an inscription on it saying it's the coffin of Genghis Khan. When the security guard is left alone with it the coffin starts to shake around and eventually opens up to reveal a man in the dress of a Mongol warrior, who telepathically forces the security guard to shoot himself. However, it turns out it's not actually Genghis Khan, but a modern-day descendant.
Maxim Horvath in The Sorcerer's Apprentice. Twice. First, Balthazar traps him in the Grimhold. He escapes. Later, they were both sealed in a Chinese vase. They both escape ten years later. And the other Morganites; Sun Lok, Abigail, and Morgana Le Fay herself.
Khan Noonien Singh and his cryogenically frozen followers in Star Trek II: The Wrath of Khan, when they're abandoned on Ceti Alpha V (which the crew of the Reliant mistake for Ceti Alpha VI after a natural disaster alters its orbit and destroys its environment).
"God" in Star Trek V. He claimed to have been imprisoned on the planet in the center of the galaxy and wanted to "join" with the Enterprise so he could escape.
In Star Trek: Nemesis, the Enterprise crew find B4 dissassembled and put him back together. It turns out he's part of an elaborate evil scheme by Shinzon.
Star Trek Into Darkness: Khan; a former Evil Overlord accused of war crimes, and his crew of Augments were cryogenically frozen for centuries in a derelict ship... until Starfleet Intelligence found him. He ends the film this way, too.
Superman and Superman II both feature the Phantom Zone, but make it out to be a one-time prison for a specific set of three villains.
The Thief of Bagdad, directed by Alexander Korda, provides a classic example, lifted from the old Arabian Nights tale, "The Fisherman and the Bottle"; Abu the thief, stranded on an island by Jaffar, finds an old glass bottle and uncorks it, releasing a huge genie, who had vowed long ago to kill whoever released him from the bottle. Abu tricks the genie back into the bottle, prompting the genie to offer the prerequisite three wishes. He turns out to be a bit of a Literal Genie.
Time Bandits: "Don't touch it — it's concentrated evil!" Also, Evil himself is unable to leave his fortress.
In Transformers, Megatron is kept on ice inside the Hoover Dam, and is pretty angry upon thawing out.
Prequel comics for Transformers: Dark of the Moon reveal Shockwave to be this; he was found by the USSR decades ago and had been sealed under Chernobyl for years. When he gets out, he's ticked off. And there's Sentinel Prime, who's been locked in stasis on board the Ark, which crashed on the moon.
The Van Helsing movie has Dracula sealed inside his own part of the world, but circumventing it by developing wings, which he passed on to his other kin.
Wishmaster is this trope. A powerful djinn sealed inside a fire opal since medieval times is awakened by the heroine who accidentally broke it. The monster will wreak havoc on earth as soon as the girl ask her proverbial 3 wishes. But she wont, 'cause that would be wrong. And so on.
In The Brass Teapot Dr. Ling knows he can’t destroy the Teapot, so he intends to seal it in such a way that no one can ever find it again. At the end of the movie he casts the Teapot into the bottom of the ocean.
A ridiculous number of these show up as non-Darklord threats to harass poor Lone Wolf.
In a slight subversion, one of these sealed evils, namely Darklord Vashna, the most powerful Darklord of them all, who even tried to play The Starscream to Big Bad Naar in the Back Story, is already dead. Not sealed alive in a prison somewhere, but dead. The goal of two of the books in the series (The Chasm of Doom and The Legacy of Vashna) is to prevent him from coming back.
Lord Zahda in Castle Death is another interesting example: he's definitely a powerful evil who had been sealed away, but not so much in a can as in a castle on an island which he ruled over, where he had subjects of his own. So a much larger, more comfortable can, at least.
At the end of book 3, The Caverns of Kalte, you are the one doing the sealing, when you exile Vonotar the Traitor to the Daziarn Plane by pushing him through a shadow gate. The Daziarn is a whole dimension of its own, not really a can, and you would eventually see Vonotar again.
The Deathlord of Ixia was a more conventional example, since the title villain really was sealed in the proverbial can, and then escaped.
Agarash the Damned, Naar's most powerful champion of evil, was sealed in another dimension by the Elder Magi millenia ago. In the Grey Star books he is the Bigger Bad goading the Big Bad Shasarak into freeing him. Grey Star has to prevent this.
Every Bal Sagoth song starts with "Oh shit, we just woke up Cthulhu." This is all the more impressive considering that (actually) awakening an elder god would crush the mind of anyone near it, then plunge the Earth into a never ending night.
"Bark at the Moon" by Ozzy Osbourne is about a beast that once terrorized a town, but was eventually buried in a nameless grave. And then he ends up returning to cause some more destruction.
This trope hearkens back to the Greek Mythology of Pandora's Box. The box iself is an unusual case, as it was created and given to her for no other reason than Zeus felt like being a dick. After seeing all the atrocities emanating from the box, Pandora quickly shut it, leaving only one thing inside — Hope. As aforementioned, all according to Zeus' plan. This is also an example of Sadly Mythtaken. The "box" was actually a jar.
The Titans were locked inside Tartarus, a dank, gloomy prison "as far beneath Hades as heaven is high above the Earth", where they were guarded by their siblings the Hecatonchires.
Typhon was trapped beneath a mountain by Zeus.
According to Islam, Dhu'l-Qarnayn, or "He of the Two Horns" (a lesser prophet who appeared long before Muhammad) encountered a race of monsters (split into two tribes, the Yahjuj and Mahjuj, or Gog and Magog of Biblical fame) that were harassing the people, so he built a giant wall to keep them enclosed. They won't be able to escape until Allah says so, which will be during the lead-up to Yawm al-Qiyāmah (the Day of Judgment).
Unbreakable magic chains kept Fenrir (a gigantic wolf so massive that it must crouch down to avoid scraping against the dome of the sky) bound, until he eventually breaks them. Ironically, it is implied that binding him is what made him so pissed off at the gods in the first place. Well, that's the trouble with prophecies... and he is a child of Loki, God of betrayal, mischief, and "evil".
The legends of genies often fall under this trope. Not all genies are good, ya see, and the good ones will interpret wishes literally. Moreover, the Ur Example for many of the bad jinn/wicked ifrit stories is the tale that Solomon/Sulayman used his ring to seal (yes, seal, with molten lead and a magic seal ring) a number of bad jinn into brazen urns (which is a fancy name for brass cans) and cast them into the ocean, only to have them later dredged up as in stories like "The Fisherman and the Genie." Since this has an evil spirit sealed in an actual can, this makes this trope 'Older Than Feudalism.'
Champions adventure The Blood and Dr. McQuark. Azor (an agent of some Eldritch Abominations) was imprisoned by the Council of Nine but escapes during the course of the adventure.
In Deadlands, a group of Native Americans sealed off "The Hunting Grounds"]] (affecting all magic, good or ill, but ill vastly outnumbered good). Centuries later, another group unsealed it, looking for revenge on the white man for killing off their tribes.
In 4th Edition, the entire plane of Baator is like this, being created by a divine curse to imprison the Magnificent Bastard Asmodeus and the devils. Unfortunately, said can is rather flimsy, and while it does a good job of holding Asmodeus, any infernalist worth his salt can create a temporary portal to it, allowing the devils to stream through and wreak havoc and contracts.
The seal wasn't made to save mortals from Asmodeus and his devils — but to save the other gods from him. Hence why mortals are free to poke holes at the seal and let devils come and do Faustian pacts. If you are dumb enough to do it, you deserve what's coming to you.
You have grasped the entire concept wonderfully. Asmodeus will be pleased.
One example is the homebrewed Harrowed class. Anyone with this class is this for their own unique destructive being that they call 'the monster within' which is constantly raging to get free from its host. Ironically if the beast were to be released it would be promptly obliterated by about seven different gods at once, this won't ever happen though because plenty of greater deities have been left scratching their heads after failing to remove the beasts. This class cannot even use any of their abilities without provoking the beast and needing to make a will save or let it gain control for a few rounds of combat.
If you think that the Harrowed are justified in wanting to die, they aren't. They are stuck in a constant cycle of reincarnation after they die and they always come back as another Harrowed with exactly the same homicidal beast sealed inside them. They are such a tight can that not even death or the gods can crack them open.
In the Back Story the current gods sealed the evil gods that ruled before them, each in a different way befitting them and tailored so that their powers couldn't get them out. For instance, Thulkas, the Iron Lord, was so strong that he couldn't be moved, so he was hammered into an arrow and shot into the sun.
The Scarred Lands also has the Slarecians, psionic beings who challenged both the gods and the titans. The twist is that they sealed themselves into the can so they wouldn't be destroyed.
In the published version of B3: Palace of the Silver Princess, a huge ruby called My Lady's Heart turns out to be a Canopener for an evil Immortal, Arik. Even the leakage of Arik's power which the gem emits is enough to petrify a royal court, trap its best defenders in a dimensional prison, and attract monsters and fanatical dark priests from hundreds of miles around.
An important part of Eberron is that there are multiple Sealed Evils in various Cans, with sealants as strong as the plot demands of them. There are three big ones, as well as a number of more localized ones:
The Overlords of the Age of Demons, aka the Rakshasa Rajahs, are the closest thing to actual Physical Gods confirmed to exist in the setting. They were big-time Abusive Precursors and were eventually defeated after a war with the dragons and locked in prisons in Khyber, where they are kept in (mostly) suspended animation. Releasing them would be a massive undertaking requiring very fine manipulation of specific prophesied events (different for each Overlord) to pop their can, as attested by the fact that they've had minions working on the problem for millennia and only managed to spring one (and that one was re-sealed in a year).
The quori, malevolent nightmare spirits, are trapped in their own dimension because it's been knocked off its metaphysical axis, preventing travel to and from other planesnote Initially, this was a case of Sealed Good in a Can, but then the Age turned- essentially resetting the entire plane and its inhabitants- replacing the relatively benevolent quori who got sealed with the current crop of borderline-Eldritch Abominations. The quori mostly get around this by projecting themselves into mortal dreams and possessing people, since while their bodies can't leave Dal Quor, their minds can...
The Daelkyr were Humanoid Abominations and the generals of an army of creatures from the Realm of Madness that invaded the mortal world several millennia ago (before the modern nations, but well after the dragon-demon war went down). They lost and got sealed with most of their minions in Khyber, though unlike the Overlords they're still awake and can move around, so long as they don't come to the surface or try to go home, both of which they're prevented from doing by special druidic seals. They do still have a disturbing amount of leeway for Sealed Evils, though.
The massive Ptolus setting by Monte Cook is set on the world of Praemal, which exists as a giant can in which to seal evil. The city of Ptolus itself is built on top of a can or ten and has a giant, impossibly high spire in it where a cleric once, long ago, tried to seal all the evils of the world in a can. Then he became evil, and eventually his fortress atop the spire was itself sealed with the remains of his work (and his remains) within it. It's a pretty can-heavy setting.
Another notable example in the Points of Light setting is Torog, god of torture and imprisonment. The kicker? The earth is his can. He's trapped in the underdark, actively crawling around down there, and occasionally reaches up from below the earth to pull whole cities down into his can with him.
In the Greyhawk setting: Zuggtmoy (in the Temple of Elemental Evil), Iuz and Fraz-Urb-Iuu (in Castle Greyhawk) and Tharizdun (in his Forgotten Temple).
In the Midnight setting there is the evil god Izrador, who was sealed (along with some other beings) in the world of Aryth. The bad part? Your characters live on Aryth and they're also sealed there. Not only is the evil in the can but you're in the can with it. Crapsack World indeed.
In the Forgotten Realms setting: one of the oldest cans would be the Sharn Wall, erected by the creatures which gave it its name so as to keep the phaerimms sealed away from Faerűn. Weakened severely over the millennia, it is later brought down by the return of the Shadovar, who then must team up with the Sharn and the peoples of Toril to drive the phaerimm back. Other examples include: the djinn Calim and the efreet Memnon, locked in eternal struggle forever, their battle creating the Calimshan Desert; Sarya Dlardrageth and the rest of her fallen house of demon-interbred Elves, trapped in the Nameless Dungeon beneath Hellgate Keep, who are inadvertently freed when the Mistmaster and his Harper allies destroy Ascalhorn's mythal with the Gatekeeper's crystal; Iyachtu Xvim, son (and receptacle) of Bane, imprisoned beneath Zhentil Keep until he breaks free in the wake of Fzoul Chembryl's reading of The True Life of Cyric; an illithid and a rakshasa that became trapped in stasis beneath Beorunna's Well after seeking revenge on Runlatha for their previous imprisonment and torture and who are even now (by using an image of the rakshasa to pretend to be the totem spirit of the Red Tiger Clan) trying to obtain the cooperation of the Uthgardt barbarians in securing their freedom; and the three nycaloths accidentally freed from their dimensional pocket by the flight of "a red dragonthat never held malice or greed in its heart over the coronal's throne"—which leads directly to the Weeping War and the fall of Myth Drannor.
The artifact known as the Acorn of Wo Mai holds an evil demigod called the Copper Tyrant of Tros that will try to trick anyone possessing the Acorn into letting it loose.
Most of the titular monsters in the 3.5e sourcebook Elder Evils qualify, with the exceptions being Atropus and Ragnorra, both of which are completely free to roam the universe, killing or corrupting (respectively) everything they encounter. Fathe Llymic is a Far Realm entity that's sealed in a glacier, the Hulks of Zoretha are slumbering alien superweapons, the Leviathan is the chaos left over from creating the world given physical form and put to sleep at the bottom of the ocean, Pandorym is a phenomenally powerful entity whose mind and body have been separated and sealed away, Sertrous is a dead obyrith whose spirit is trapped in his skull, Kyuss is a would-be god who is sealed in an obelisk on an isolated island, and Zargon is the previous ruler of Baator, who was imprisoned by Asmodeus in a giant rock under a desert.
Dark Sun setting. After the Champions of Rajaat realize that Rajaat's Cleansing Wars are actually attempted genocide, they seal him in a prison dimension.
Dragon magazine #46 adventure "The Temple of Poseidon''. The Cthulhuoid Dark King Ythog-Nthlei (AKA Zoth Ommog) was sealed into a sarcophagus hundreds of years before the adventure begins. Unless the PCs can stop it, he will be released during he adventure.
In the prehistory of Exalted, the defeated Primordials were stitched into the twisted body of their god-king Malfeas and confined to another realm... mainly because the Exalted looked upon the Primordials who had been killed, seen that they'd brought the Underworld into existence, and said, "Yeah, that ain't supposed to happen."
There are also the Neverborn, the aforementioned Primordials that had been killed. Being too big to die properly, they instead wrapped themselves up in giant sarcophagi spanning infinite distances as they resisted the pull of Oblivion. Then the Solars decided to pop a few blocks open to learn Necromancy...
From the perspective of the Bronze Faction, this was the case for the Solar Exaltations locked in the Jade Prison for ~2000 years.
In the In Nomine universe, one of the prophecies for Armageddon is "An ancient evil breaks its bonds." One of the possibilities for fulfilling this is Magog, Demon Prince of Cruelty, who was sealed in an ancient Egyptian tomb exactly one day after falling from Heaven.
In Iron Kingdoms the Skorne have Ancestral Guardians, which are statues empowered by the souls of fallen Skorne warriors.
In the game Neuroshima there are a number of items scattered around the world, which adventurers refer to as Pandora's boxes. Each is a large metal canister that contains something nasty (from a biological weapon to a very compact assassin droid). Everyone knows what they are but humans being humans, every once in a while someone opens one, either because of the Schmuck Bait involved or in some desperate gambit against his enemies.
Pathfinder gives us Rovagug, the "Rough Beast" and god of destruction. He resembles a giant arthropod and seeks nothing more than to undo the works of the other gods and destroy creation itself. Faced with this foe, good and evil deities alike joined forces to destroy him, and many lost their lives. In the end it was Sarenrae and Asmodeus who managed to take him down, with Sarenrae battling him for long enough that Asmodeus was able to seal him within the world of Golarion with a special key. Rovagug is still there within the world, where his struggles are blamed for volcanoes and earthquakes. The Pit of Gormuz (a 20 mile wide chasm) is a weak spot in his prison, and while Rovagug himself can't get out, he can periodically release some new Spawn on the world to continue his work (the most famous of which is the Tarrasque).
A common trope in Pathfinder, Golarion has many hidden cans of evil, some of which now breaking free due to the recent death of humanity's patron deity. Runelords, ancient tyrannical wizard-kings, sealed themselves away to escape an ancient catastrophe. The elves trapped the plant-corrupting demon Treerazer in their nation. The Whispering Tyrant, the world's most infamous necromancer, lies imprisoned by an entire nation of paladins. Many published modules and adventure paths involve the player characters stopping some sealed evil from being unleashed — "Realm of the Fellnight Queen," "Pact Stone Pyramid," and "From Shore to Sea" just to name a few.
Scion, by the same company as Exalted, uses the same logic surrounding the Titans. At the end of the Titanomachy, the gods bound them away in the Underworld — this was because the Titans were incarnations of things such as Light, Fire, Darkness, Life, and Water, and their death would screw with reality big time. This was proven when Ymir was killed... causing the Great Flood, as the Ice Age ended right then and there.
Daemon weapons contain bound daemons within them, giving them incredible power and just as incredible malevolence. However, the greatest threat isn't them escaping, but rather corrupting the weapon's wielder and twisting them to its will.
More powerful daemons, who couldn't at the time be banished back to the Warp, are often sealed in the Materium in some way that prevents them from killing everything at the moment, perhaps in the hope that they can be banished later. This being WH40K, not only does the sealing frequently make things worse, but most of the time those doing the sealing never tell anyone about it.
Both literal and metaphorical in the case of the Necrons and their C'tan masters (later retconned to have actually been shattered by the Necrons and chained), who sealed themselves deep beneath the surface of numerous worlds approximately sixty million years ago.
The new Necron superheavy unit, the Tesseract Vault, is essentially an attempt at weaponising this trope by taking a C'tan trapped in a high-tech device to keep it caged, and then having that device float around the battlefield dropping meteors on people. In an emergency, the Nemesors can disable the seals and let the C'tan loose.
During the Great Crusade, the Emperor sought to gain access to the Webway. In a top-secret chamber deep beneath his palace on Terra, he constructed a massive machine that allowed him to breach it. He had to put some heavy-duty psychic wards around the place to keep daemons from using the gate to enter Terra. Unfortunately, Magnus the Red broke those wards to tell him of Horus's betrayal. Since then, the Emperor has sat on the Golden Throne, using his psychic might (and the might of thousands of daily psyker sacrifices) to keep that gate closed.
The Uktena from Werewolf: The Apocalypse make a trade of this; one of their camps, the Bane Tenders, possesses secretive rites that allow them to seal away Banes too powerful to kill, so that they can't get out and wreak havoc on the world.
This is also the grand view of the Wyrm, whose evil was not the cause of his sealing, but vice versa; the Weaver, herself a bit batshit at the time, decided to trap the Wyrm within her webs, driving it mad and shifting its purpose from "a blessed end to all things" to "reality cancer, forever and ever." Some of its most devout cultists seek to undo the Weaver's webs so that the Wyrm can be set free. Unfortunately, since those webs make up a good chunk of reality...
BIONICLE: Makuta Teridax encased in a Toa Seal by the Toa Metru by their Elemental Powers, but much later they unintentionally released him with the same powers when they attacked Roodaka holding a piece of the Toa Seal.
Charby the Vampirate had a big old , slightly subverted since it had originally been sealed by an amaturish effort to defeat it, only to reseal it with a tighter lid later after it got loose. We didn't learn that evil came in cans until after the fact.
In Girl Genius, the Other is a being of such extreme evil that it wants to enslave the world... and doesn't really care who dies in the process. It was eventually defeated, but nobody knows HOW... except maybe the parents and uncle of the protagonist. It left a machine behind with a copy of itself, which it then imprinted on the protagonist (unsealing it), who is the daughter of the Other, or rather, taking into account certain implications and statements that lend themselves towards it having been an actually ancient evil, it simply possessed the protagonists mother, and hundreds of other people through time, before finally taking possession of the protagonist, temporarily.
The Axe of Prissan in Goblins acts as a prison for an immensely powerful demon lord, whose very existence threatened the mortal plane. The enchantments binding the prison will gradually break down if the axe is not used for good, or even faster if used for evil. As such, it has enchantments layered on it to ensure it will aid any paladin who encounters it and refresh the prison.
In Schlock Mercenary, it looks like Oisri is a massive prison for the dark matter entities like the ones which destroyed the galaxy in the alternate timeline. Petey thinks it's something far worse.
Inverted in The Way of the Metagamer: the evil is sealed within the Book, and does not escape... but the heroes are forced to follow it into the Book.
Subverted in 8-Bit Theater. Red Mage places the fire demon Kary in a Bag of Holding and freezes it with a powerful Ice spell, with the idea that she will remain sealed until they are powerful enough to defeat her. White Mage simply smashes the frozen bag in revenge for the death of Black Belt.
BEN of Ben Drowned is within a Majora's Mask Nintendo 64 cateridge, though how he originally got inside is relatively unknown.
Neopets now has its very own Sealed Evil In A Can, as of the end of the Return of Dr. Sloth plot event that happened January 29 - March 15, 2008. The comic is here, and spoilers are here: Dr. Frank Sloth is the sealed evil. The can is the Space Faerie's token. Roll your mouse over the very last panel of the last chapter, and you'll see the token, which was not destroyed in the explosion of the ship, floating through space. When it gets most of the way across the panel, a pair of red eyes glow from within...
Phantasia has more than one of these, of varying degrees of world-destroying power.
Many, many things contained by the SCP Foundation. One, SCP-076-2, had the particularity of coming with his own (leaky) can, SCP-076-1, and the SCP (after many Pyrrhic Victories keeping it in the can) eventually decided to try and work with him; predictably, it didn't end well.