Evolutionary Pressure Cooker

"You ever heard of the bug urn? One way they whip up poison out in Chena. Basically, you fill an urn with a bunch of poison bugs, have 'em eat each other, and use the last survivor to make your poison. ... Harsh truth, but the Lintons did that shit with people. Grabbed promising kids up from all over, dosed 'em all with poison, and fed the ones who died to the survivors. I was the one who lived. The poison bug who survived on the flesh of his siblings. That ain't a metaphor: I literally survived a bug urn."

— Tim Linton, Reign of the Seven Spellblades Volume 10

Create a container. Fill it with all kinds of dangerous, deadly creatures. Seal the container away so that the starving creatures have no choice but to feed on one another or die. When you open the container sometime later, just what will you find?

In real life, you're likely to find a bunch of dead creatures. Any that aren't dead will most likely be dying of starvation or malnutrition, or weakened and injured. In fiction, however, you may very well find that one survivor has changed into something far more powerful, and deadly, than all the others together.

This is the process of the Evolutionary Pressure Cooker. Seal countless creatures, people, ghosts, etc. away so that they will survive on one another's death, until one (or few) survivor(s) remains, leaving it (or them) changed. The end result is typically far more powerful, almost certainly deadly, and frequently much more ruthless and/or evil.

The specific method behind the effect may vary. In some cases, it could just be a natural effect, defined as an inevitable result of such a scenario with no further explanation. In others, it could be explained as a form of severe enforced Level Grinding, or forcing the survivor to assimilate all the abilities/power of those it killed, in a manner similar to how some poisonous creatures absorb the poisons of their prey.

This trope frequently occurs in Eastern media, in part due to the existence of a curse in ancient Chinese folklore, known as a gu (蛊, lit. poison) or jincan (金蠶, lit. golden silkworm), a name for the resulting creature, in China, and a kodoku (蠱毒, lit. curse poison or worm toxin^note not to be confused with the more common homonym 孤独 kodoku meaning "solitary/lonely") in Japan, which used insects and other poisonous creatures in an attempt to focus the poison.

May involve Monstrous Cannibalism (if creatures of the same kind eat each other), and Cannibalism Superpower if the winners absorb the strength of the losers. See also Sole Survivor, which governs all scenarios where only one survives without the necessity for individuals to survive by killing or cannibalizing one another. Frequently appears in the form of a Battle Royale Game. Compare to Super Breeding Program, which relies on selective breeding to produce more powerful offspring. Is often used by Social Darwinists. Some biological Death Worlds can become analogous to this, but on an ecosystem scale.

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Examples:

Anime & Manga

• The 100 Girlfriends Who Really, Really, Really, Really, Really Love You: Played for Laughs in Chapter 96. The chapter starts with Hahari buying an expensive vase that she thought Yaku would like, which turns out to be too big to fit inside the school building. She ends up stuck inside it along with Iku, Kusuri, Kishika, and Momiji, which the narrator dubs as a Gu ritual.
• Blue Lock utilizes this concept to produce a powerful striker, trapping the best amateur strikers in Japan in a building and forcing them to compete against one another, with all defeated banned from professional play, so that the desperation to survive at the cost of others will both further hone their skills and force them to develop a ruthless, self-centered mentality the organizer believes necessary for the strongest striker.
• Dark Gathering: The concept is referenced by Yayoi during the creation of High Priest of the Evil Sutra graduate-type. When the room became imbalanced due to Unnamed Kami's attack, all remaining ghosts start to eat each other until only one remains, which gets promoted to a special weapon. Otogiri the Soul Sucking Oiran was initially just a haunted mirror until she has evolved in a similar manner.
• Dokumushi: The manga centers on an attempt to create a Kodoku using murderers sealed in a house with preparation tools for butchering and preparing meat but no food besides one another.
• Hunter × Hunter: The succession arc is based on this. Guardian Beasts are summoned through a "seed urn" inspired by "worm toxin" (one translation of kodoku), and the heirs who've become possessed by these beasts are then left on a ship to kill one another until only one remains.
• Inuyasha: Big Bad Naraku, a demon with a poison/corruption theme, was originally created when a wounded bandit whom Kikyo treated in a cave lusted after her, and called for nearby demons to give him the power to make her his. The cave de facto became a kodoku, with the demons devouring him and each other to birth Naraku as a conglomeration of them all. Naraku later attempts to create a powerful new body for himself through a more literal kodoku by sealing demons within a mountain and forcing them to fight it out.
• Jujutsu Kaisen: The Zen'in clan keeps a room of cursed spirits that they use to punish and humiliate noncompliant family members. When Ogi leaves Maki and Mai there to die, he only half-succeeds: Mai passes away and takes her cursed energy with her, allowing Maki to evolve and kill the rest of the creatures.
• Kengan Ashura:
  • The series has a martial arts equivalent of this as the final part of training under the other Niko Tokita, directly evoking a comparison with the gu ritual — all students are locked in an underground cell with enough food and water for one person to survive for 30 days. The purpose is for Sole Survivor to be one person who managed to "evolve" and surpass other fighters, killing them in combat. Kanoh Agito is a survivor of such experiment, who was rescued before Niko could collect him after 30 days and finish his training, while Lu Tian is the one who finished it. This method is deconstructed in that the former needed years of therapy to become functional and spent even more still dealing with the split-personality disorder he gained as a result of this trauma, while the latter is just flat-out Ax-Crazy.
  • Kengan Omega reveals that the Inside, the lawless region within Japan, was created to be a large-scale version of this same ritual.
• Kodoku Experiment involves a dying planet full of monsters desperately fighting for survival, creating a form of "kodoku", with the final survivor, part of a crew that was deliberately left on the planet, transforming into a bizarre lifeform that infects those around it.
• The second series of the Medabots anime introduces Blakbeetle as the sole survivor of a competition to the death among 100 Kilobot prototypes.
• Mushijo involves arthropod-like creatures known as "Maji" that possess humans in order to claim and eat a certain human in order to reach evolution.
• Naruto Shippuden: Torune Aburame's Jar of Poison Technique in episode 317 traps poisonous insects in a pit with piles of corpses, in imitation of kodoku. The insects are forced to multiply into a gigantic swarm that is then unleashed as a Fantastic Nuke upon the surrounding landscape.
• Resident Evil: Heavenly Island: Three guesses about what the "Kodoku Project" entails. The island is kept completely isolated to utilize this technique to perfect a powerful bioweapon, with normal inhabitants brainwashed to destroy any means of escape.

Comic Books

• Superman: This is part of the origin of the monstrosity Doomsday, as seen in Superman/Doomsday: Hunter/Prey. Its creator, Bertron, would launch a baby out in ancient Krypton's harsh environment and if the creatures outside didn't kill it, the atmosphere would. Bertron would do this over and over and over to create the Ultimate Life Form.

Film — Live-Action

• Piranha 3D: This is the explanation that the marine biologist Carl Goodman gives for how the titular piranha became such savage predators. They are a prehistoric species that was trapped in an underground cavern millions of years ago, where they cannibalized each other in a process that made them bigger and meaner until they were finally released by an earthquake opening a passage into the lake.
• Skyfall: The Big Bad Raul describes utilizing a similar method to deal with rodent problems. He traps rats in oil drums until they are forced to resort to cannibalism, then releases the two survivors. The last two are strengthened by the effort and, more importantly, conditioned to continue killing and eating other rats. Raul compares this to how M and the Secret Intelligence Service train and treat their operatives. The conversation gets a Call-Back in the form of Bond's remark upon killing Raul: "Last rat standing."

Folklore

• As stated above, Far Eastern folklore dating back at least as far as the 7th century includes the creation of a curse or poison using this method on poisonous creatures. The result is an unkillable creature that has many applications according to records:
  1. The excretions of the creature form an absolutely lethal poison. The wealth of those killed with this poison is always drawn toward the Kodoku's master. The bodily fluids can alternately be used as ingredients in various concoctions to control a target.
  2. The Kodoku naturally brings wealth to its master. However, it demands to be regularly fed human sacrifices, or otherwise it will devour its master instead.
  3. The only way to be rid of it is to place a large amount of gold and treasure, including the worth of all wealth gained from the Kodoku, along a street for another to pick up. This may be used as a form of curse, giving someone a valuable gift with the Kodoku hidden inside, so that the recipient, unaware that they'd been gifted this creature, would fail to make the sacrifices and be devoured themselves.
• A proverb first attested in Greek by the 15th-century Byzantine writer Michael Apostolius, and subsequently quoted by various Renaissance-era writers, posits that a "[a] serpent, unless it devours a serpent, will not become a dragon."

Literature

• Blood Angels: Deus Encarmine features a flashback to protagonist Rafen and his friend Arkio entering the Gladiator Games used by the Blood Angels chapter to select their aspirants. They were two of the survivors, battle-hardened by the experience, but still ridden with cancers that the conversion into Space Marines cured.
• Bofuri: I Don't Want to Get Hurt, so I'll Max Out My Defense.: The original Japanese curse is referenced in a quest. Poison-using player Maple is examining "antiques" in a shop on the fourth floor when the shopkeeper offers to show her an urn. She doesn't read the quest before accepting but hears the shopkeeper say that he hopes she'll be the last one as she's sealed within the urn together with a horde of powerful bug-type poison-attribute monsters. True to the poison nature of the curse, she is rewarded with a latent skill that gives all poison attacks a 10% chance of instant death regardless of the opponent or their resistance.
• Chrysalis (RinoZ): Performed in layers. The colony requires monster cores, but demons tend to evolve without developing cores, as the energy required is expended fighting for survival. To solve this, larvae are spawned in cells that only open after a core is detected, so they grow stronger killing one another, then the survivor has time and freedom to build a core. The cell opens to progressively larger containers, repeating the process of forcing the monsters to grow by killing one another and then enhancing their cores until the strongest reach the top and are harvested.
• "Crabs on the Island" is a short story by Anatoly Dneprov (adapted into an animated short film, The Crabs), in which a scientist creates an army of metal-eating, self-replicating crabs and sets them against one another, hoping their capacity for Mechanical Evolution will eventually produce an ultimate weapon.
• Dracula: Renfield, who is obsessed with the idea of devouring "life force", experiments with forcing collections of animals to devour one another to better concentrate that life force and create a better "meal".
• Ghost Hunt: A high school overrun by ghosts is eventually discovered to have been set up as a form or curse using the Kodoku method, mixing an Ouija board with a Chinese curse talisman and taking advantage of the spiritually isolated location to gather countless spirits and force them to devour one another until one (or, in this case, four) is powerful enough to carry out the intended deathcurse.
• Reign of the Seven Spellblades: Master Poisoner Tim Linton reveals in volume 10 that the magic tradition of the Lintons is to scoop up magically talented children and repeatedly poison them and force-feed the victims to the survivors until only one is left, which they then raise as their heir. Tim directly compares this to the "bug urn" practice used in Chena, and the process understandably left him severely messed up: when he came to Kimberly Magic Academy, he found it so similar to what he'd survived that he attempted a mass Murder-Suicide by setting off a Deadly Gas bomb in the cafeteria. Alvin Godfrey saved his life, and Tim has been devoted to his cause (and hopelessly in love with him) ever since.

Tabletop Games

• Exalted: This is a story of how Luna came to be. Oramus and Cytherea pitted all the potential moon-gods against each other, to see which would triumph. Luna won, seducing or destroying her rivals, and became Creation's sole moon-god(dess).
• Warhammer 40,000:
  • Some Space Marine chapters, notably the Blood Angels and the Black Templars, use Gladiator Games or other combat tournaments to select new aspirants: the survivors come out battle-hardened fighters that can then undergo the conversion and training into Super Soldiers.
  • Done accidentally by Lord Inquisitor Kryptmann during the Third Tyrannic War. He managed to trick one of the major wings of Hive Fleet Leviathan into invading the Ork Empire of Octarius, hoping they would either destroy each other or at least buy the Imperium some time. Unfortunately, it resulted in the Hive Mind absorbing ork DNA, resulting in even stronger tyranid organisms emerging from the star cluster when they finally destroyed the orks. Ultimately it took a Deus ex Machina at the Siege of Baal to finally end the war.

Video Games

• Digital Devil Saga: The six tribes that inhabit the Junkyard are directed to fight each other and feed off each other's strength by the Karma Temple at its center, promising that the one tribe that emerges on top will ascend to Nirvana. In reality, the entire situation is taking place inside a computer program designed to find the "optimal" configuration for urban warfare.
• Fate/Grand Order: The 15 Bespectacled Intellectuals event references the kodoku by name when explaining the event's monsters eating each other to make the survivors grow stronger in an attempt to kill the heroes.
• Final Fantasy XIV: Omega forces the Warrior of Light (and friends) to participate in a gauntlet of battles against digital recreations of powerful enemies from fiction and alternate realities to determine why The Hero is able to defeat more empirically powerful Big Bads. It hopes to determine what this unknown factor is to add it to its already dangerous arsenal of abilities obtained from fighting greater foes through self-improvement protocols. Although Omega doesn't physically trap the heroes in this experiment, it forces them to participate by threatening to perform a global massacre by spawning monsters all across the star if the heroes don't comply.
• Mewgenics: Your own house, where you can breed and raise your cats, can become an example of this on a smaller scale. Each day, a fight might break out between two of the house's occupants, with the outcome of the fight influenced by their stats: if there is a clear victory, the loser is maimed or killed while the winner gets stronger; thus, cats that win fights are likely to win more fights, until you have a vicious powerhouse of fur on your hands.
• This is the basic premise of the obscure DOS game Unnatural Selection, in which you breed mutant critters in order to try and create stronger ones to fight the mutant creatures a Mad Scientist is creating. You subject your creatures to various mutagenic effects like drugs and radiation, then breed them to see if you can create stronger monsters.

Webcomics

• Mob Psycho 100: Matsuo captures spirits in a "kodoku jar" (translated in some places as simply a "poison jar") to have them devour one another so that he can keep the last survivor as a powerful "pet".

Real Life

• Atomic gardening was a practice in The '60s where seeds were planted around a source of gamma radiation. The ones next to the source died, the ones close to it had hideous tumors, but those far away had their genes modified to a non-lethal extent by the radiation, and were grown in order to determine if these genetic mutations were of interest (weather/pest resistance, size, crop yields, etc.)
• Marine biologists have begun using a variation of the approach called "assisted evolution" in attempts to save the world's coral reefs from human-caused climate change: by rapidly breeding more heat- and acid-resistant coral polyps and symbiotic algae in controlled environments, then releasing the specimens into the reefs, they hope to save one of the ocean's most fertile biomes from collapse.

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