In Age of Mythology the cyclops Gargarensis is on a mission to free Kronos from Tartarus in return for godhood. He fails. Kronos gets out in the expansion, but gets buried right back in by Gaia.
A Mess O' Trouble (an excellent Mac WorldBuilder shareware Adventure Game) has two godlike creatures trapped inside time dilation bubbles in some ruins. You know from local historians (and abominations lying around in the ruins) that their civilization was practically constructed by a good creature and then fooled into nearly destroying itself by a bad creature. One is a beautiful Energy Being, the other a dull-looking lizard man. Guess which is which?
The hook and most the line of Arcanum's plot involved the player character being a Chosen One prophecied to defeat a Big Bad, last known to be sealed in a can. Later it's revealed that many evils are sealed in that can, and by the time you finally wormhole your way inside, the Big Bad has done a Heel-Face Turn long ago, after having been overshadowed by an Evil Overlord you must defeat instead.
Arcanum: Of Steamworks & Magick Obscura has the void, an alternate dimension where the Elven Council banished 5 Evil beings: A demon with insatiable hunger, the murderer who killed Arcanum's last dragon, a genocidal barbarian king, and two corrupt wizards. The player character has to undergo banishment so they can destroy the Big Bad, Arronax Kerghan, before he can regain his freedom.
The entire Arc The Lad series revolves around this trope with the Dark One. He starts every game being sealed, winds up being unsealed at some point, and then gets re-sealed by the end of the game.
In the first Avernum / Exile game, there is the Haakai Lord Grah-Hoth, who was sealed in a bottle before the game begins. Despite being unable to directly act, the game reveals him to be manipulating events, such as aiding the Slith against the humans who imprisoned him.
In the Expansion Pack of Baldur's Gate 2, the Bonus Boss is the avatar of a very powerful demon lord. You are asked to reseal him in his prison as he's about to break free. Alternatively, you can fight him in an epic battle. However, if you win, you realize that demon lords in Dungeons & Dragons come Back from the Dead very, very quickly by definition. So it will happen, though not for the rest of the game. Good going.
Well they don't actually die, destroying their physical form returns them to their home plane, in his case it frees his avatar back into the abyss to be reabsorbed by the real one.
Kangaxx the demi-lich is another example. He dies very permanently after underestimating you, however.
The Bard's Tale, being a parody of fantasy RPG tropes in general, has two of these. The first is the Nuckelavee, which our hero accidentally releases. The other is the "Princess" that he's supposed to be rescuing. Turns out that she's really a powerful demon, and the world gets progressively worse as her seal weakens.
The Bard seems to make a habit of unsealing evils. There's also a dragon, which the Bard had apparently set free before the events of the game. One Side Quest brings him to the town that it ransacked, and the residents are none too happy.
Boktai 2: Solar Boy Django has a textbook example in its Big Bad, Jormungandr.
The Japan only sequel has a similar deal, except its cosmic horror, Vanargand, was sealed on the frickin' MOON.
In Borderlands, the Vault only contains—as far as the player is concerned—a monster called The Destroyer who is "immortal in his own realm". Sadly, this final fight is easier than a single monster from the previous zone. The Vault also opens every 200 years, which raises the question of it really is a sealed can.
A significant portion of the plot of Brave Fencer Musashi revolves around this. The first major quest in the game involves Musashi being sent to collect Lumina, the legendary greatsword used to defeat the Wizard of Darkness; a significant portion of the rest of the game revolves around Musashi questing to recover the elemental Scrolls in order to power Lumina up enough to take out the Thirstquencher Empire. Unfortunately, it turns out the voice which set Musashi on this quest in the first place, a voice originating from Lumina itself, was actually the Wizard of Darkness himself, not destroyed but sealed within the sword. Guess what the only things holding him back are? If you guessed "the five Scrolls and the Crest Guardians they're connected to," you win the kewpie doll.
Dracula's castle in the Castlevania series is eventually sealed this way after the Dark Lord's death. It's sealed inside the total solar eclipse of 1999. In the solar eclipse of 2035, it sucks in everyone present at the shrine where the sealing took place.
Dracula, and Castlevania itself, were also sealed in the underworld throughout most of the series, both of them reappearing on Earth only once every hundred years. But Dracula found so many ways to circumvent that rule that it became more of a "sealed evil in a sieve".
A subversion, Lavos was an alien that burrowed into the earth and then unleashes the apocalypse many ages later as part of its natural life cycle. Essentially, it seals itself in the final can and breaks out when it wants. The villains of the game attempt to awaken him to harvest his power, with the apocalypse being a byproduct of this.
The City of Villains is practically filled with these: Bat'Zul under Cap Au Diable, the Leviathan under Sharkhead Isle, Shiva in Bloody Bay... and the City of Heroes isn't lacking in them either, as Dark Astoria apparently houses the sleeping dread god of the Banished Pantheon, and the Kaiju that may still be in battle with Talos underneath Talos Island... Also, both sides can get involved in the escape of the Reichsman, Nazi with the power of the gods.
The Undying King from Clive Barker's Undying. who was actually the LID on the can, opening the door to the rest of the series. Unfortunately, this well done game bombed monstrously, so the series never materialized...
The Relic of Moirai in Contra: Shattered Soldier, which is revealed to be the mysterious force that the alien attackers in the previous Contra video games were trying to recover the whole time, after it was taken and hidden away by the Triumvirate, according to Lance Bean.
In Dark Cloud, the general of an expansionist empire frees the Dark Genie from its place of captivity. At first, it seems to grant his wish by destroying every other nation on the face of the planet, but in the end, it takes over his body to progress towards its final goal — the complete destruction of everything.
Jedi Knight: Jedi Academy: The sealed evil takes the form of an ancient Sith Lord, whom the game's big bad is naturally trying to resurrect, thinking she will be rewarded. The Sith Lord has other ideas.
In Dark Messiah Of Might And Magic the Demon lord is locked in a prison under the city of Stonehelm, however the original prison is flawed and can be broken, the player can choose to get the good ending and seal him away forever by repairing the seal or get the bad ending by releasing him.
Dawn of War, the original one, has an absolutely beautiful example. An ancient demon imprisoned in a stone manipulates Orks into attacking human cities on the planet. Space Marines come to the rescue and do what Space marines do best - spill ungodly amounts of Orkish and human blood - just so at the end, the demon can reveal that the entire planet had previously been converted into a blood altar for releasing him. Bonus points: most characters wanting to use demon or its power for their own end up badly.
The Chaos Rising expansion to Dawn of War II also features a plot to unseal a demon trapped inside a planet that was lost in the Warp.
In Dead Space, the Marker seals the infection that turns corpses into horrific alien monsters (it was actually a manmade knockoff of the real marker). Anyways, the Marker-Worshipping ScientologistsUnitologists discover the Marker in the midst of mining Aegis VII and move it off its pedestal. Bad things occur.
The Devil May Cry series. Every. Single. Installment. ...except the fourth. That one just needed to be awakened rather than unsealed and at the end of the first one Mundus is resealed in the can..
In the Diablo series, when the Prime Evils were unleashed upon Sanctuary, the Archangel Tyrael selected a group of mortals known as the Horadrim and charged them with imprisoning their essences into the Soulstones so that they would not be reborn into the Hells upon death. But thanks to the betrayal of Tyrael's lieutenant Izual, who filled the Prime Evils in on the Soulstones and how to corrupt them, it turns out that this played directly into their hands.
The demons in Doom 3. Sealed in a can until Dr. Betruger teleports himself and the Soul Cube into Hell. They were sealed back into the same can at the end, as well...
Played with in Dragon Age: Origins. The Darkspawn seek out the seven ancient dragon gods buried within the earth, and when they find and awaken one it becomes the Archdemon and leads them in a massive invasion of the surface world - but the sealed gods were not necessarily evil until tainted by the darkspawn, making them more like Sealed Badass in a CanGone Horribly Wrong.
The draconic Old Gods of the Tevinter Imperium in Dragon Age: Origins were, according to the Chantry, banished to the depths of the earth by The Maker to slumber for all eternity. The Darkspawn are somehow able to hear the Old Gods' Call and devote centuries of effort tunneling through the earth in search of them. When they finally discover an Old God, the Darkspawn taint immediately corrupts the ancient dragon, turning it into an insane and twisted shell of its former self — an Archdemon. The new Archdemon then commands the Darkspawn hordes in a bid to kill everything — a Blight. By the time the game starts, the world has already suffered through four such Blights. One of the biggest secrets that the higher-ups of the Grey Wardens keep from the rest of the Order is that they know where the Old Gods are buried.
In a DLC of Dragon Age II, Hawke finds an old Grey Warden fortress, which turns out to be a prison for one of the original Darkspawn - a Tevinter mage who attempted to reach the Golden City and was cursed by the Maker. The magic seals have held for thousands of years, but are finally starting to give out.
In Dragon Age II Hawke can fight several powerful demons that were sealed away in and around Kirkwall. The "Legacy" DLC revolves around the Hawke family's connection to an ancient Sealed Evil In A Can Corypheus one of the original Tevinter Magisters who brought the Darkspawn Taint to Thedas whose subconscious efforts to free itself have brought danger to Hawke's doorstep. There's also ANOTHER uber-monster sealed right outside the prison housing him, but that one's an optional Bonus Boss.
Dragon Quest VIII has Rhapthorne, with two cans: the scepter of Trodain to hold his soul, and the statue of the Goddess on Neos to hold his body.
The Grotesqueries in Drakengard, with a twist. Also, no one knows the Sealed Evil in a Can exists except possibly the Big Bad. They're concerned about some other thing that comes out when those seals are broken.
In the final route of Duel Savior Destiny instead of killing Lobelia after she is beaten, her soul is trapped via alchemy when she tries body hopping.
Dwarf Fortress has what is popularly referred to in the community as "Hidden Fun Stuff". If your dwarves tunnel far down enough, they may breach a secret chamber containing demons which are powerful enough to bring the fortress to its knees. It's possible to kill them, though.
As of the latest update there are now "Demonic Fortresses" which are a bit like the pits, but the pits have been replaced with hell itself, which you simply reach by digging far enough, implying most of the physical structure of the planet is a caverns made of an unmineable, indestructible, impossibly heavy Unobtainium who's exits are blocked by another Unobtanium which is very valuable, light, and hard. And every Demonic Fortress contains a hole straight to Hell, blocked by a masterwork sword made of the latter type of Unobtainium. As for Hell itself, it contains so many demons that the game doesn't bother counting them.
EarthBound: Giygas is first encountered in the Devil's Machine, which seals away his warped consciousness. Subverted in that Giygas can still damage Ness and his friends while sealed away, but played straight when Pokey unleashes Giygas' true form by turning the machine off.
Earth 2160 has the traditional sealed-ancient-evil-alien-race-beneath-the-surface-of-Mars for the first half of the game. Then some Dutch nerd learns to control them, and it all ends badly(-er).
The main storyline of The Elder Scrolls III: Morrowind focuses around preventing Dagoth Ur, a godlike being imprisoned for 3500 years, from fully regaining his sealed powers.(though he wasn't imprisoned but passed out)
Likewise, in its followup, Oblivion, the player must prevent Mehrunes Dagon, lord of the titular realm, from unleashing his forces upon the world. Dagon isn't really sealed IN anything so much as he is kept OUT. In this case, the world is the Can Sealed Against Evil.
In The Elder Scrolls V: Skyrim, the Sealed Evil in a Can is Alduin. And he isn't unsealed by someone playing around with something they know nothing about, but was an inevitability since his sealing involved him being cast adrift outside of time. A prophecy describes the events that must transpire before he would end up back in linear time again.
In the Infocom text game Enchanter, your job is to defeat the evil enchanter Krill without disturbing the Cosmic Horror that's sealed below his castle. The tie-in novel by Robin Bailey takes the tack that your character accidentally did release the thing, and now it's up to the book's protagonist to stop it.
Akron, the Final Boss of Epic Battle Fantasy3. He notes in his Boss Banter that he has been defeated and sealed away countless times through time immemorial — but in the end, his enemies all succumbed to time while he always returns.
In Evil Zone, the inhabitants of an island dimension sealed away an incarnation of a cosmic destroyer, but couldn't finish the job themselves, so they had to hire out heroes to finish the Big Bad off.
The entire point of most the rituals in the Fatal Frame series is to make sure the sealed evil (specifically Hell itself) stays that way, though naturally they always eventually fail and plunge the area into endless suffering,we'd have no game series otherwise
Most Final Fantasy games feature a Sealed Evil in a can. Even Final Fantasy Tactics, which is grounded in realism and political intrigue, has an evil god trying to find a suitable host body.
Final Fantasy III gives us the Cloud of Darkness, the living essence of the power of the Dark (as opposed to the Light of the heroes' world.) The game implies that Xande's machinations allowed it to take form, but it would have remained sealed away in the Dark World had it not been for him opening a portal leading straight into it. The Dark Warriors imply that they fought the Cloud 1,000 years ago, when it was Light surrounding Darkness, and got it Canned within the Dark World. Xande was nothing but a can-opener.
Zemus in Final Fantasy IV, who, despite being sealed in the Lunarian Moon was still able to influence events in the world in an almost successful attempt to effectively kill all humans. Notably, he is never released from his can; you raid it.
Final Fantasy V's previous generation of heroes, the Braves of Dawn, used the power of the Crystals to seal away Exdeath, who then surreptitiously began to drain the power of the Crystals (either personally or through manipulation.) Additionally, the player learns that, prior to Exdeath, the sorcerer Enuo was the first to harness the power of the Void, and waged war with it until he and all his obscenely powerful demons were thrown into the Dimensional Rift. The Advance version expands upon this by letting the player explore Enuo's prison and vanquish him for good.
Also in Final Fantasy V there once was a tree in the Great Forest of Moore used to seal up evil spirits. Eventually, the power of those spirits gave the tree sapience, power, and a whole lot of evil. That tree became Exdeath; a prime example of sealing so much evil away that the can itself turns evil.
The Warring Triad of Final Fantasy VI, who started the War of the Magi, sealed themselves away after realizing the destruction they had wrought upon the world, and the Espers hid them away in their own underground kingdom. Then Kefka came and released them, destroying the balance of magic and devastating the world. It also released Humbaba, Deathgaze, and the eight dragons who are released during the apocalypse.
Jenova of Final Fantasy VII is a basic example of the trope; she spends most of the plot going from tomb > water tank > freezer. In the sequel movie Advent Children, what's left of her is stored in a literal can.
In Final Fantasy VIII, Laguna Loire tricks evil sorceress Adel into walking into a specially-prepared technological "tomb" in which he is able to seal her considerable powers. Then he launches it into space and spends the next seventeen years making regular trips to monitor the seal. Predictably enough, catastrophe eventually lets her loose again, but by that point Laguna's son is well-equipped to kill her off for good.
Shuyin of Final Fantasy X-2 is a special case. In one sense, he is still sealed within the Den of Woe, but in another sense he is free to wander around within the body of first Nooj, then Baralai. Also, while his goal is indeed to break free of his prison and destroy the world, this is because he's a Woobie, Destroyer of Worlds, not an Ultimate Evil.
The Tonberry enemies in the Final Fantasy series also appear to be like this. Despite their goofy appearance, their Grudge attacks suggest that they're like vessels for all the malice and wrath of slain monsters.
Fire Emblem: Path of Radiance alone uses the trope straight, with the characters believing that Lehran's Medallion contained an evil god that flooded the entire world save Tellius. So at the end not only do the heroes fight Ashnard in order to liberate Crimea, but also to prevent him from unleashing a dark god. However the true nature of the relic is revealed in the sequel Radiant Dawn, and the heroes are the ones who free Yune, the Goddess of Chaos, rather then letting the negative energies of war do it. As it turns out, Yune was actually Sealed Good in a Can who just happened to mess up (the Great Flood); in fact, the Big Bad is Ashera, the goddess everyone had been praying to throughout both games, who purged her emotions (forming Yune) to avoid another Great Flood.
The Dread Lords from Galactic Civilizations 2. The Dread Lords had a pocket dimension which they had used as a base for their fleets. Their enemies, The Arnor, used this against them by sealing the last of their empire in the dimension. Later on, the Dregin found the lock to the Dread Lord prison. Thinking that is was an ancient weapon, they activate the device, allowing the Dread Lords to escape and wreak havoc across the galaxy. Upon seeing this, the Drengin realize their mistake and decide to leave the others for dead.
Well before the start of the Geneforge series, the Shapers discovered a startling new technology that could imbue ordinary humans with incredible magical powers. When they discovered some of the side effects involved (such as Suicidal Overconfidence, a violent temper, and in extreme cases, outright Body Horror), rather than take any steps to destroy this technology, they simply abandoned the remote island outpost where it was discovered, and declared it off limits under penalty of death. Fast forward a few hundred years: A band of explorers from across the seas happens upon the abandoned outpost and all its forbidden goods. Things go downhill from there.
In fact, the Shapers do this constantly. Their laboratories, workshops, and schools are designed to be sealed up quickly should anything Go Horribly Wrong.
Not just should. It's said at one point that more often than not something does go wrong.
In Guild Wars, Palawa Joko suffers this fate. Then there's Abaddon and everyone else locked in the Realm of Torment, including the Titans from the first game. Ultimately, in an attempt to stop Varesh Ossa from opening a gate to the Realm of Torment, you have to let Palawa Joko out of his prison.
In the Halo series, the bad guys accidentally release the Flood, a race of alien parasites that were sealed in special facilities all over the galaxy at the end of a cataclysmic war between them and the Forerunners 100,000 years ago. This war ended with the extinction of all sentient life in the galaxy, so it's a wonder why the Forerunners left little pockets of Flood spores for nosey aliens to stumble across. The AI monitor of one of these facilities comments on this (while you're in the middle of fending off a large wave of rotting space-zombies), saying that specimens were kept over after the last outbreak "for study," and remarks that "this decision may have been in error." No shit...
Done again in Halo 4 with The Didact, who is most likely the last living Forerunner, being released by the Master Chief.
Coincidentally, Homeworld: Cataclysm also involves a similar scenario. Somewhere around a million years ago, the extragalactic exploration vessel Naggarok picked up a deadly technoorganic entity in hyperspace. Seeing no way to defeat it, the crew scuttled the engines, trapping the entity in deep space. However, they screwed up as the ship auto-launched az empty lifepod with a transmitter (and some Beast material) onboard. In the present, the Kiith Somtaaw mining ship Kuun-Lan finds the pod and opens it. Cue to a race with time to find both the Naggarok and the new Beast mothership and blow both to smithereens before the whole galaxy suffers a fate worse than death (ship crews aren't simply killed, they're broken down into biomass to function as a makeshift neural interface between ships and the Beast - and judging by the sound of it, it's not exactly painless).
Kirby Squeak Squad plays with this one a little. What started as a hunt for stolen strawberry shortcake leads to Dedede getting smacked down on false suspicions, Kirby chasing all over the world to get his snack back from the titular menace only for the chest allegedly supposed to hold the shortcake stolen away by Meta Knight, and when HE gets smacked down, the Squeaks grab the chest and let loose Dark Nebula. For such a simple protagonist the plot for these can get quite complex, especially given Meta Knight grabbed the chest away just to prevent Dark Nebula from being released. The best part is that through all of this, you get the feeling that Kirby is still being motivated only to retrieve the cake.
seems to be the case with the Kingdom of Sorrow in Klonoa 2: Lunatea's Veil. As it turns out, it was sealed away because no one wanted to remember sorrow, and the King of Sorrow tried to undo the seal so everyone could remember sorrow.
The Legend of Kyrandia III: Malcolm's Revenge has the player play as a Sealed Evil in a Can, who is rather dismayed to discover that being unsealed does not include getting his awesome magical powers of doom back, leaving him running around with no powers in a fantasy kingdom where everyone hates his guts.
And there was also Bongo Bongo, the Evil Shadow Spirit from ''Ocarina of Time' that was sealed in the Bottom of the Well.
The Legend of Zelda: Spirit Tracks features demon king Malladus, who tyranized the land that would one day become New-Hyrule when Tetra and Link arrived. He was sealed by the ancient Lokomo spirits underneath vast chains that eventually developed into a railway-system.
In The Legend of Zelda: Skyward Sword, the God of Evil Demise was sealed by the goddess Hylia. His servant Ghirahim who is actually Demise's sword seeks to free him and succeeds. In the end, Link seals away Demise again within the newly forged Master Sword.
Lightning Legend: Daigo no Daibouken has Dragless, a legendary and all-powerful Demon King who rampaged through the Kingdom of Japone 450 years before the story's proper, destroying everything on his way. He ultimately was defeated and sealed away in the deepest ends of Mt. Ohsore by the young hero Taikei Raioh, but by the start of the game he has finally freed himself, and it's now up to the descendant of Taikei, Daigo, to defeat him for good.
In the LucasArts computer game LOOM, Bishop Mandible unleashes Chaos by ripping open the fabric of the universe near a graveyard.
In Luigi's Mansion, major ghosts that Luigi encountered were once imprisoned inside portraits, but were released by King Boo once E. Gadd captured Boolossus. Eventually they are recaptured by Luigi and put back in their portraits, including King Boo himself... although in the next game he has apparently been released to fight Mario. He is killed this time and next time he is fought (in this case by Peach).
The world of The Magic Candle was narrowly saved from the immortal demon lord Dreax when a few heroes managed to trap him in a candle flame. Keeping him there is the daily task of 44 mages... who have just disappeared. The seal is now critically weakened, leaving the player a set number of days to find out how to fix it.
Somewhat unique in that the point of the game is to reactivate the seal, not simply to grind yourself to the point where you can just kill the damn thing (because doing so is impossible, at least by the terms of the game world).
Parodied in Makai Kingdom as Zetta seals himself in a book after he destroys his own netherworld. Hilarity Ensues as he tries to get his body back.
Marathon Infinity starts out with a grim message from Durandal about the W'rkcacnter getting loose from Lh'owon's sun, due to the Pfhor using the trih xeem on it. The W'rkcacnter cannot be fought directly, and is only defeated by the player jumping between different places and timelines, before the player reaches a Jjaro space station that is able to turn the sun into a black hole, thus trapping W'rk before it (them?) escapes.
The Dark Star from Mario & Luigi: Bowser's Inside Story. It was imprisoned in Toad Tunnels until it is released by Fawful. Its core is destroyed by the Mario Bros. and its main form is killed by Bowser.
In Mario & Luigi: Partners in Time, at the end it is revealed that Princess Peach seals Elder Princess Shroob, the leader of the Shroobs, inside the Cobalt Star. When the star is pieced back together, she is released, and fights the Mario Bros. She is defeated, and eventually killed when she possesses Bowser and the Mario Bros. trick him into hitting her with his fireballs while aiming for them.
The Reapers in Mass Effect actually seal themselves after their habitual galactic genocides in order to conserve energy. The only hand the Protheans had in dealing with them was tampering with the Citadel's mass relay after they'd already receded so that they couldn't get back out so easily.
The DLC "The Arrival" reveals that the Reapers left themselves another way in. A special mass relay at the edge of the galaxy that can access any other relay anywhere.
The Antarans from Master of Orion 2: Battle at Antares. They are an ancient and powerful race who were banished to another dimension by the Orion Empire. However, they learned how to break out of their prison and were able to warp raiding parties to attack colonized worlds while preventing others from invading their prison-turned-stronghold. Nice job breaking it, Orion.
The Shadow Demons in MediEvil were sealed in the heart of the Enchanted Forest under an iron dome, locked with the Shadow Artifact. In order to get through the forest and to his next destination, Sir Dan Fortesque is forced to free them; he later makes up for it by trapping them in an abandoned castle and dropping it into lava, destroying them.
The "can" in question is in itself a Sealed Evil, although, becoming a hero, this was obviously subverted.
In Metroid Prime: Hunters, "the ultimate power" broadcast in a telepathic message throughout the galaxy is actually the sealed evil Gorea, originally a Giant Space Flea run amok and the presumed source of the message. This is one of the few cases where the good (the Alimbics) weren't strong enough to kill the evil, just entangle its energy with theirs in the Seal Sphere and hide it in a pocket dimension. You arrive at the Seal Sphere only to see the other Hunters pounding away at it like idiots. They break it open, then stare as the core of the Seal Sphere (the Alimbics' energies) is snatched by Gorea, who proceeds to impale all of them with tentacles from the Sphere and steal their energies (weapons). You saw this coming, or at least you WOULD if you've scanned at least 25% of the Alimbic lore hanging around the place... Anyway, this leaves you to clean up the mess (kick Gorea's ass).
The titular creature from the first Metroid Prime game was sealed within he impact crater. In the original version, the Space Pirates managed to free it by digging under the seal, although it later escaped from them and returned to the crater, and you have to open the seal to fight it. In the European release/Player's Choise version, it was never released.
In the Mortal Kombat series, the fallen elder god Shinnok is sealed in the Netherworld. The Realms themselves are actually the Can keeping the One Being sealed. Merging them would allow the One Being to return, which is why the elder gods resist any attempt at unification.
Mother 3: When his mech runs out of power, Pokey/Porky retreats into his Absolutely Safe Capsule — which can't be opened from the inside or outside. Not quite SEIAC, as this capsule cannot possibly be opened, damaged or otherwise altered by any earthly or divine means.
Speaking of Bungie, the main plot of Myth: The Fallen Lords is this, and it's reanimating corpses and whatnot. After you all but lose the war, you manage to kill it.
A more literal example is The Watcher, a powerful Lich who was imprisoned in a cave by a charm on his hand that would turn him to stone if he tried to leave. Ultimately, he chose to cut that hand off. He finally met his doom when an arrow was fashioned from the bone of the hand that was left behind and turned him to stone after striking him.
The demon of the first Ninja Gaiden game, and the Archfiend of Ninja Gaiden: Dragon Sword.
In Okage: Shadow King, King Stan has been (purportedly) stuck in a bottle for the last 300 years, waiting for someone wimpy enough to let them possess his shadow. Additionally, while he was stuck in the bottle, a number of monsters stole portions of his evil power and became "Fake Evil Kings". He then drags the main character around to defeat them and get his powers back so he can take over the world.
Orochi in Ōkami is a definite example of this trope.
In what is possibly the worst-sealed can ever, Pac-Man. He kills the ghost, sealing it in the little box in the center of the screen. Three minutes later, it escapes again, and poor Pac must kill it over again. Perhaps he'd have better luck if he gave his little ghostbox a lid.
In Paper Mario: The Thousand-Year Door, the plan of the antagonists is to summon the Shadow Queen, who can only be summoned into a princess (read Peach). Upon release, the Queen promptly reduces the head of the antagonists to, well, a head, and plans to conquer. Unluckily for her, Mario arrived at the same time.
On the other hand, she recognizes Mario's strength and offers him to work for her. The player is given a Yes or No choice, while the former leads to an instant Nonstandard Game Over.
In Super Paper Mario, Bonechill, a fallen Nimbi, was held in the area of the Underwhere in which the dead villains suffer for eternity among the game overed. He escapes due to an earthquake (often speculated to be the work of the Big Bad, Count Bleck, who is known to be a dimension traveler) and wreaks havoc in the Overthere. Mario, Peach, Bowser, and Luigi fight him and easily destroy him.
Antasma in Mario & Luigi: Dream Team. About a thousand years ago he was sealed away in the Dream World by the Pi'illo people, who ended up getting turned to stone by his last act. And then, when everyone finds out Luigi can open Dream Portals by sleeping... he manages to capture Princess Peach, and then escape back to the real world just a short while later. Guess that can was ludicrously ineffective then...
In Pathways Into Darkness, the Marathon games predecessor, a modern-day Special Forces team must prevent a W'rkcacnter from escaping from its can.
The Snow Queen Mask in Persona is definitely one. It's kept in a gym storage room, in a box sealed with MAGIC TALISMANS, and after going on That One Sidequest in which everyone warns you about the horrible past of said mask, you can just decide to open the box like nobody's business and walk off with it completely unpunished.
The Dark One of SRMTHFG has nothing on the Profound Darkness in Phantasy Star IV; the Precursors sealed it with a solar system.
In Phantasy Star I, II, and III, you find the big bad end boss Dark Force/Dark Falz/Dark Phallus (depending on translation) in a literal Pandora's Box in the final dungeon.
In IV, you discover that the Profound Darkness is one of these, and it has been creating Dark Force/Falz/Phallus for thousands of years (please note that there's a thousand year gap between I and II, and between II and IV'').
In Phantasy Star Online 2, you learn early on that Dark Falz (One of its incarnations, at least) was sealed beneath the surface of the planet Naberius a few decades before the game's story takes place. Unsurprisingly, it ends up being released sooner or later.
Phantom Brave features an example as the main antagonist, the demon Sulfur. Rather than the usual thousands of years of imprisonment though Sulfur is capable of coming back every 30 years, and during his imprisonment can extend enough influence into the world to wreak havoc.
In Planescape: Torment, the literal Sealed-Evil-In-A-Box. Also, the Monster Jug (which you can buy at a shop and open, should you feel like it).
In Prototype, Elizabeth Greene, the host of the Redlight Virus is sealed in the Genetek building. Badly. The protagonist might also count as a Sealed Evil In A Can, although in this case it's more like Sealed Evil in a Vial.
Quest for Glory is fond of this one. In fact, every game following So You Want to Be a Hero centers around such a plot.
Toward the end of Trial by Fire, it is revealed that Ad Avis is trying to summon Iblis, a powerful and evil djinn.
In Wages of War, the Demon Sorcerer attempts to free the Demon Lord. (If he succeeds, the Demon Lord's first act is to cast Thermonuclear Blast on the immediate area. As it turns out, this is a legitimate spell, and can be learned in the fifth game.)
In a minor twist, in Shadows of Darkness, Avoozl, the Dark One, wasn't quite sealed properly, and the surrounding countryside has suffered for it. Even as the two antagonists (one new, one old) try to release it, it is only through their actions that it can be put away for good.
Dragon Fire has a twist of its own - there is a villain working behind the scenes and trying to unleash the Dragon of Doom, but by this point in the series, the hero (under extenuating circumstances) has become strong enough at this point to just kill the thing.
The Tyrant in ''Resident Evil', which is released by Wesker near the end of the game. In the bad ending, it escapes from the facility.
In Resident Evil: Code: Veronica, an experiment with the Veronica virus on Alexander Ashford mutated him into a grotesque monster codenamed Nosferatu, who is imprisoned in Umbrella's Antarctic base until our heroes arrive. Alexia also counts somewhat.
In Resident Evil 4, the parasitic Las Plagas were sealed away for eight generations before the start of the game.
Heinrich I, in Return to Castle Wolfenstein. Doubly so, given what he does to Blavatsky. Earlier in the game, there's Olaric, who is accidentally unleashed by Helga von Bulow when she tries to take the Dagger of Warding.
Return To Krondor presents the Dark God as this. The Dark God does not get released, but the ending makes it clear that the person trying to unseal it has not given up.
In Romancing SaGa: Minstrel Song, the god Saruin was sealed away via the Fatestones; naturally, his minions are trying to 'correct' this. However, an even better example is the Jewel Beast: a monster poised to destroy a whole country if awakened. Even if the player manages to delay its awakening — no easy feat by itself, given the precise timing and difficult sequence of events that involves — they can't stop it unless they enter its lair while it's still sleeping. It's one of the harder fights in the game.
In Rune Factory 2, mysterious earthquakes start happening about a third of the way into the game. Eventually, the main character's child finds these are being caused by a Sealed Evil in a Can which is about to escape and destroy the world.
RuneScape: The devastating Zarosian creature Nex (Latin for "violent death") has been sealed into a enchanted, frozen sleep by the armies of Saradomin. Sliske tricked a band of peace-seeking Saradominists into performing a ritual they thought would wake Guthix from his slumber, but instead released Nex.
Averted and parodied in Septerra Core. The game's intro movie and backstory tell about a great battle in which Marduk (the world's Crystal Dragon Jesus) defeated Gemma (the local Satan equivalent). In most RPGs, at some point towards the end Gemma would be resurrected and become the final boss. The main character even speculates that this is going to happen after hearing the tale about the battle between Marduk and Gemma. In response, The Obi-Wan remarks that such a plot twist would be rather silly, and only happens in stories. Sure enough, Gemma never comes back, and the final boss of the game is the character who's been the main villain from start to finish, the Knight TemplarEvil Overlord Lord Doskias.
In Shadow of the Colossus, except there are sixteen cans wandering throughout the area. Most notable with the final Colossus though. Besides having a name that literially means 'evil' in Latin, the only possible way you can reach it is to have killed all fifteen other Colossi ingame. Might also apply to Dormin as well, if you're part of the group that thinks They really are evil.
Seems to be somewhat of a recurring theme in the Shining Series. Evil guys try to unseal something worse;
Shivers has the Ixupi, ancient soul-sucking Mayincatec demons who were sealed into pots with talismans. Millenia later, an itinerant archaeologist digs them up and puts them in a museum. Then two kids sneak in the museum and break them open.
Sonic the Hedgehog (2006) has the sun god Solaris, forcibly split into two seperate forms due to an experiment acted upon it: Iblis and Mephiles. Princess Elise served as Iblis' can, with her control over her sorrow being the lid — if she cries, the can is opened and Iblis is unleashed upon the world once more. Somewhere else, Mephiles is sealed away in the Scepterof Darkness until an encounter with Eggman shatters it.
Shadow the Hedgehog made his first appearance as this, though being canned and going evil didn't happen at the same time: his creator screwed with his memories after he was placed in stasis, leading to his whole 'humanity needs to die' outlook.
Sonic Unleashed has Dark Gaia, who was sealed within the planet by his light counterpart a.k.a. Chip in a neverending cycle of planetary death and rebirth.
Malefor, aka The Dark Master from The Legend of Spyro series. He was born as a purple dragon, like Spyro, some countless generations ago. He was taught how to master the elements, but kept gaining more power, resulting in the Elders banishing him to exile. He took on the title of The Dark Master, where his malice was so great that it split the Earth, creating the Mountain of Malefor, also known as the Well of Souls, where he was imprisoned. In A New Beginning, he sends out Cynder to open the convexity portal to free his soul, which she succeeds in, though Spyro frees Cynder from his control, causing the portal to implode. In The Eternal Night, Gaul uses the lunar eclipse of the celestial moons (that causes non-stop darkness for a short while) to seemingly resurrect Malefor. But it's revealed by Malefor in Dawn of the Dragon that this was merely all a ruse to get the real one to free him to the Well of Souls to do so, who was it? Poor Spyro.
Also, the Destroyer counts, as its sole purpose for existing is to destroy the world but it slept below a volcano until Malefor awakened it to destroy the world. A rare case of one Sealed Evil In A Can freeing another.
In the Touhou game Perfect Cherry Blossom the evil cherry tree, the Saigyou Ayakashi, is sealed up by the dead body of Yuyuko Saigyouji. At the same time, the tree seals her, preventing her from reincarnating, so she wanders as a ghost. In a wierd twist, Yuyuko eventually forgets her life as a human, up to and including why she died or who is sealed beneath the Saigyou Ayakashi. This leads to her trying to undo the seal that she placed in the first place, because she forgot she did it.
The player characters in Undefined Fantastic Object think that the incident is releasing one of these, however Byakuren is very much a Sealed Good in a Can.
From the perspective of the Buddhists, the Taoists in Ten Desires are this, what with that whole religious war thing. As usual for Touhouthey aren't evil, but neither are they particularly good.
Turok 2: the objective is to stop the Cosmic Horror Primagen from being unsealed from his can.
In Valis II, Cruel King Megas had been sealed away long ago, but was released when rebels battling the forces of the fallen Lord Rogles broke the seal and opened the Forbidden Door. The result was to turn Vecanty into Hell on Earth.
Vampire: The Masquerade – Bloodlines features a memorable subversion — the MacGuffin of the game is a sarcophagus that supposedly contains an Antediluvian (very old, very powerful vampire). Everyone is trying to make a move for the sarcophagus, partially because the presence of such a thing might be a herald of Gehenna — but also because a low-generation vampire represents a massive power grab for anyone willing to commit diablerie (the consumption of another vampire's soul). Well, the Prince has been pushing you around all game in an attempt to claim the sarcophagus, and when it's finally opened... the only thing he finds is a lot of C4. And a note from the guy who set the whole thing in motion. Boom.
Illidan Stormrage in Warcraft III, another Blizzard production, is imprisoned in a cage for 10,000 years for continuing to research arcane magic after the night elves had banned its use. Also a subversion in that Illidan is not evil at the time of his imprisonment, but has become obsessed with power and revenge by the time he is freed.
Maybe Illidan wasn't evil, but he did kill people with a handwave who tried to stop him from corrupting a lake with the Well's water.
In a continuation of this universe, a majority of raid bosses in World of Warcraft are sealed evils. The quests to kill them generally go something like Go beat up these mildly bad dudes who have this Big Ancient Evil imprisoned, so that you can kill him too. One wonders why the player doesn't just say But, they're doing a fine job keeping him imprisoned! What happens if I manage to kill them but the Big Ancient Evil kills me? A variation goes Go beat up these mildly bad dudes who are trying to unseal this Big Ancient Evil before they succeed, then kill the half-unsealed form of the Big Ancient Evil, which makes a little more sense.
Warcraft's universe also has the Old Gods, very similar to Lovecraft's Great Old Ones, sealed beneath the world and waiting to be freed. For the longest time, it was a total mystery why the god-esque Titans didn't just kill them all, considering they'd managed to off one. Recently it was revealed that the Old Gods are parasites who have bonded with the planet of Azeroth so that killing them will cause untold damage to it. And yet, the players continue killing them for loot...
The fourth expansion to World of Warcraft (Cataclysm) involves the unleashing of Deathwing, the Earth Dragon who was slowly driven insane by the Old Gods and imprisoned in Deepholme, the elemental plane of earth. His emergence not only blows up parts of the world, but also opens up the elemental planes, which are full of Sealed Evil in a Can, including the elementals themselves (locked up because they were tearing the world apart with their wars)
The Mists of Pandaria expansion features the sha - beings of pure hatred and other negative emotions - as this. It later emerges that an artifact called the Divine Bell has the ability to control the sha, and Horde Warchief Garrosh Hellscream orders it stolen in an attempt to make super-soldiers with it. It misfires when the sha ends up controlling its test subjects instead of the other way round, and have to be slaughtered by adventurers. The sha are eventually revealed to be fragments of a slain Old God whose still-living heart is removed from a Titan prison and used by Garrosh in another attempt to empower his loyal orcs.
Wario Land games have had a few cases of this, with Wario Land 3 having Wario spend half the game trying to 'help' a mysterious figure trapped in the music box, who turns out to be Rudy the Clown, which then tries to take over the world. Wario World had him accidentally unleash the sealed evil in a can at the very beginning, aka the Black Jewel, which was taken from some kind of treasure chest by Wario and his obsession with treasure, and that then turned his entire castle into a parallel dimension of sorts and what not.
Might and Magic VII has a rather simplistic sealing liable to be done by the player characters: the medusa crawling around in the sub-level of an abandoned mine have developed an immunity to magic. As a class promotion quest, you get sent to sabotage the elevator to keep them down there.
All games in the Ys series use this trope, e.g. Darm in I and II (who was disguised as the Black Pearl, also an Artifact of Doom), Galbalan in III, the Ancient City and Arrem in IV, the lost city of Kefin and its king, Jabir, in V, and the Ark of Napishtim in VI.