The use of Life Energy or a soul, usually of human origin, as a type of Applied Phlebotinum. That is, a functional component or fuel of a machine.
If these machines are used by people other than the Big Bad or his Mooks, you should take this as a sign you're dealing with a Dark Fantasy world where Gray-and-Gray Morality is the norm.
A Sub-Trope of both Soul Power and Magitek. It also may overlap with Powered by a Forsaken Child and Haunted Technology.
Examples:
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Anime & Manga
- Souls quite literally have power in Fullmetal Alchemist. In the manga, the Philosopher's Stone turns out to be human souls condensed into energy. In the 2003 anime, the human souls of the dead in our world serve as the fuel source for alchemy itself.
- In the Mobile Suit Gundam side story The Blue Destiny, the super system EXAM is powered by the soul of a trapped Newtype girl named Marion Welsh, trapped there by its creator who feared Newtypes as a whole.
- Evangelions in Neon Genesis Evangelion are powered by souls, basically making them three human-like beings in one (which is definitely a problem sometimes). Typically, they are the soul of the pilot's dead mother (also weird).
- Psyren has Amagi Miroku using the life force of every living thing that dies as his power source.
- In Puella Magi Madoka Magica, the magical girls' magic gems, called soul gems, are in fact their souls, removed from their bodies, that can be used to detect witches. Every time they use their powers their Soul Gems darken, turning them into a witch when it goes fully black, so they need Grief Seeds, which are souls of defeated witches, not to fuel their powers but to clear their soul gems and to prevent their becoming witches. Furthermore, because witches are magical girls who have gone evil, every grief seed is effectively a soul gem. Thus, this trope is played straight and inverted at the same time with the same thing.
- In Tengen Toppa Gurren Lagann, the Spiral Energy can do pretty much anything, and it's the "scientific" representation of a person's soul.
Comic Books
- Sonic the Hedgehog (Archie Comics): For a time, Dr. Eggman utilizes this in the form of Egg Grape Chambers, in which he traps live Mobians and/or humans and uses their life force to power his city and technology. The process is anything but pleasant; Ixis Naugus is driven completely insane, Charmy Bee suffers amnesia, and thousands of others, especially the Echidnas, die.
- In one issue of Stormwatch, it's revealed that Heaven and Hell are warring extradimensional engines powered by human souls (which are actually electromagnetic biosignatures) and the only way to escape them is to die in a nuclear reactor so that the electromagnetic pulse disrupts your essence.
Literature
- In The Dresden Files, soulfire, the celestial counterpart to hellfire, draws upon its user's soul as a power source and can kill them if it's overused. However, as long as it's used in moderation the soul can recover from this with time (more quickly with the right emotionally "recharging" experiences to speed the process along) without suffering actual lasting damage. In fact, it's implied that repeated use actually strengthens the soul, allowing it to be used more in the future.
- This is the premise of The Gasp by Romain Gary. A scientist discovers a way to trap and harness 'something' that's released when people die, but being an atheist, he refuses to believe it could be a "soul".
- The titular castle in Hurog is one of those. It is Powered by a Forsaken Child, or maybe forsaken adolescent. Though not technically an engine, it is a self-repairing castle, with inbuilt magical servant/slave. The hero of the novel and current owner of the castle is not happy about this, but the only option to free the forsaken boy is to kill him.
- In InterWorld, HEX's ships' engines are literally powered by the souls of captured walkers. The process involves first putting the soul in a jar and then using it as a power source. Both processes are excruciatingly painful.
- In the short story "The Last Run" by Alan Dean Foster, a "Banzai Runner" — a member of a Southern California subculture who drive an assortment of Cool Cars in very illegal ultra high-speed street races on the highway — winds up facing off against the Devil himself, in the form of a mysterious new contestant who drives a car "so black it was almost purple" with red headlights. In the end, the car is of course "powered by souls".
- Nightside features a suicide bomber who's had his soul transformed into a Fantastic Nuke that could damage the fabric of The Multiverse. He eventually has a change of heart, but, unfortunately for him, the process is irreversible. Julien explains:
"Blow an atom apart, and you get a lot of energy. Blow a soul apart, and you get the kind of energy, the kind of explosion, that can blow holes in reality itself ... There are those who see it as the ultimate form of suicide. Destroy your soul, and you get to cheat Heaven and Hell."
- The metal killing machines in the Protector of the Small series use the souls of children to power them. The necromancer who creates them doesn't have to use children, but he actually enjoys invoking Powered by a Forsaken Child.
- The Screwtape Letters: Demons work hard to tempt humans to be damned to Hell for the entirely practical reason that souls are demons' food.
- A slight variant in The Stormlight Archive, where the fabrials are powered by the sentient ideas known as spren.
- The Sunlit Man: All technology and vehicles on the world of Canticle are powered by the sunhearts, which are souls that have essentially been melted into lumps of solid Investiture by exposure to the planet's supercharged sunlight. Even the heroes use sunhearts, since nothing else provides a power source potent and compact enough to keep ahead of the sunlight.
Live-Action TV
- The sixth season of Supernatural has the angel Castiel trying to garner 50,000 souls to power some sort of angelic nuclear bomb. It's repeatedly stated that souls are basically nuclear reactors, which is why it takes some doing for God to forge them, and that an Angel's immense power is largely due to their Grace channeling this power.
- In Torchwood, Death is said to have needed thirteen souls to walk the Earth permanently.
Tabletop Games
- GURPS Technomancer: The Soul Burner Gestalt is a necromantic device that converts the souls of sacrificed human victims into magical energy.
- In Warhammer 40,000, the God Emperor of Mankind is kept alive thanks to the daily sacrifice of untold numbers of psykers, allowing his spirit to continue lighting the Astronomican without which interstellar travel is impossible.
Video Games
- In Asura's Wrath, Mantra is the fuel that powers the demigods' technology and grants them their superhuman powers. The demigods can draw Mantra from human prayer. It's also far more efficient to just drain human souls for it. Over the 12,000 years between their betrayal of Asura and his resurrection, the Deities have gathered trillions of souls' worth of Mantra, much of it being prepared for the Seven Deities' resident Wave-Motion Gun, the Brahmastra, which they intend to use against Gohma Vlitra.
- In BlazBlue, the Nox Nyctores are revealed to have been created using a ton of human souls. Hazama also needs more souls to activate the Cauldron and create Kusanagi.
- Dark Souls:
- This is heavily implied. Most sorcery spells are soul-themed reappear from the games spiritual predecessor, Demon's Souls, and considering that leveling up is again done by absorbing souls, it seems likely that this is once again the fuel for sorcery.
- The First Flame/Flame of Disparity is this. It would have gone out at least one thousand years ago had Gwyn, Lord of Sunlight, had not turned himself into its fuel source. If the player chooses the Link the Fire Ending, they become the new fuel source.
- In the second game, both King Vendrick and the Ivory King worked out a way to create golems that are powered by souls. They're just lifeless statues until an enemy is killed near them, at which point they absorb the soul and start moving. This could be a good thing (maybe the golem is the mechanism by which that heavy gate is opened) or a bad thing (maybe the golem will now start attacking you itself).
- In the third game, Soul of Cinder in a manifestation of whoever linked to the First Flame, it is an Animated Armor powered by the poor souls burning inside for eternity, including the Chosen Undead and Gwyn, the only ways to spare them from burning are to either put off the First Flame for good or to usurp the fire.
- In Demon's Souls, magic, named 'the Soul Arts', is this. Arcane magic is based on understanding and uses soul energy as the energy source, while witchcraft directly channels the power of a demon's soul.
- Doom Eternal: It's eventually revealed that Argent energy, used by multiple civilizations as a solution to their energy crises, is in fact made from processed souls (human as well as alien lifeforms), refined in Hell itself.
- The Elder Scrolls:
- The series has the ability to steal the souls of beings and trap them inside gems as a power source. First, the player has to use a Soul-Trap spell (or a weapon enchanted with that effect) on an enemy and then kill them. The defeated foe's soul is then stored in a Soul Gem, assuming the player has one of appropriate size in their inventory. There are six types of Soul Gems, depending on the type of soul being trapped: Petty, Lesser, Common, Greater, Grand, and Black. The first five apply to creatures or humanoids that aren't classed as playable/sapient (giants and the like), increasing in power depending on size. Black Soul Gems are the equivalent of Grand Soul Gems, but are the only gems capable of trapping sapient "black" souls, i.e. the races you can play as. Filled Soul Gems can then be used to enchant items to give them magical effects, or to recharge enchanted items.
- Skyrim has automatons ("animunculi") built by the Dwemer which carry filled Soul Gems. The larger (and more powerful) the automaton, the more powerful the Soul Gem typically is. This has led to the theory that these automatons are powered by the Soul Gems, which would make them literal soul powered engines. (There is debate among the fandom in regards to this, as previous appearances by the Dwemer animunculi did not carry the filled Soul Gems.)
- In Fall from Heaven, there's the 'Soul Forge' Wonder, an unholy engine that uses souls to speed production. In game-terms, this means that any unit that dies within a 1-square radius of the city, is added to its production (enemies and friends alike). If combined with Mokka's Cauldron (a Wonder that causes any unit that dies in the city to immediately be reborn as a Demon), a city can become essentially impossible to conquer, spawning units faster than you can kill 'em.
- Used periodically in the Final Fantasy series.
- Final Fantasy VII had the ShinRa and it's mining/extracting/whatever of "Mako Energy". Turns out that "Mako" comes from the "Lifestream", which is essentially a soup of human souls floating around on the inside of the planet.
- In Final Fantasy IX, the Big Bad blocked the Well of Souls that takes the dead to the afterlife, leading to the backed-up souls piling up in the form of dense, magically potent Mist. Not knowing the source, people naturally began using this Mist as fuel for magitechnology and airships.
- Machine Hunter: this is the method used by the eponymous Machine Hunters to capture defeated robots; their body physically transforms into an aura of electrical energy that posses the machine, regenerating the machine's health to maximum and allowing them to control the machines against enemy foes. They can also jump from one robot to another (defeated and immobilized) robot, but by doing so the previous robot their soul inhabits will self-destruct.
- In Street Fighter, M. Bison's Psycho Power is powered by his own, weaponized soul. Doubly so with Rose's canonically opposite Soul Power.
- The main character of Suikoden has the Soul Eater, a True Rune that drains the souls those close to its bearer to empower itself.
Web Animation
- Dreamscape: Drake's Spirit Runes, which is a person's identity extracted into a stone. Creating one is completely harmless to the target.
- Dingo Doodles: The parties mechanical dragon is powered by two gems one draws power from the sun, the other sucks up nearby souls and uses them as fuel. If it's running low on souls, it will attempt to refuel itself, through mass slaughter.
Webcomics
- Sluggy Freelance has a Haunted House that turns out to actually be an elevator to hell, with ghosts in the gas-tank! (Quoted verbatim.) Essentially, a number of captured spirits are used to power a spell that opens a gate into hell — and back — in order to provide a magician with an escape-clause from his Deal with the Devil.
Western Animation
- The Duchess from The Adventures of Puss in Boots uses the souls of wizards as ammo for her special gun, which allows it to fire said wizard's signature spell as her own.
