You've just run out of your source of magic, whether it's Magic Points or other Phlebotinum, and you desperately need to cast a spell to save the day. What do you do? Spend your own Life Energy on the spell in place of whatever would normally power it. The spell is then Cast From Hit Points.
The cost of the life energy thus expended will vary. In extreme cases, a spell cast from HP may cost the caster their life, resulting in a Heroic Sacrifice; this is often done for the purpose of Taking You with Me. When done by multiple casters at once, this qualifies as a Combined Energy Attack. Lesser versions may result in a decreased lifespan, which is typically given in round numbers such as years. Whether this is cut from physical longevity or some kind of cosmic clock depends on the series. At its mildest, casting from HP leads to immediate physical effects such as fatigue or a Psychic Nosebleed. Casting from hitpoints in a way that causes irreversible/cumulative damage to the caster is Power Degeneration, while fueling a Super Mode from hitpoints is a Heroic RROD.
The effects of this on the magic itself vary as well. A spell cast from HP may work normally, but more often than not the plot demands that the use of life itself must amplify the effect dramatically. If done well, this may represent the caster's Crowning Moment of Awesome.
Some fantasy settings have this as their standard system of magic. In those cases, the process will typically exact a price significantly less than the life of the caster. Particularly common in settings featuring magic which has limits and obeys scientific (or pseudo-scientific) principles to some degree. A common form of the Dangerous Forbidden Technique if the costs are exceptionally steep.
Not every Taking You with Me spell involves casting from HP. A parting shot may hurt the caster, but unless it is the act of casting that damages the caster, it doesn't qualify as casting from HP. When there is no way to recover at all from the loss, it is then Cast From Lifespan. You can usually be healed after you Cast from Hit Points (and thus get the hitpoints restored). If the sacrifice is of mental rather than physical health, see With Great Power Comes Great Insanity.
Compare Living Battery. Polar opposite of Mana Shield.
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Anime and Manga
The Caster's three most powerful shells in Outlaw Star, #4, #9 and #13, work at the cost of the user's life force. Firing two will leave the user winded, and a third will bring them near death. "Just like, you know..." Gene fires four in a short period of time.
In Dragon Ball and Dragon Ball Z, the line between ordinary attacks, Cast from Hit Points and Cast From Lifespan is blurry; characters use their ki to fire attacks, which is roughly analogous to their fighting power. Energy attacks can cause a huge exertion on the user's body, leaving them out of breath or weakened.
Tenshinhan's Tri Beam (and Neo Tri Beam) draws on his life force to attack with, something that inherently adds to an attack's potency, as shown by its effect on Cell. In a straight fight, Tenshinhan's power was nothing compared to Cell's, but by using his Tri Beam, Tenshinhan was able to hold off Cell long enough for 18 to escape, but then collapsed from the exertion.
Goku's Kamehameha also seems to draw on his energy reserves a lot, as shown in his fight with Perfect Cell; After blowing Cell to pieces with the attack, Goku's power level dropped dramatically.
Goku's transformation into a Super Saiyan 3 also serves to do this; the physical strain on his body of maintaining the state and using energy attacks whilst in it is enough to drain him of all his energy in a very short time.
In the games, though, energy attacks are derived from an external Ki meter separate to health, with few exceptions.
In the Budokai games, the Tri Beams are one of those exceptions. The Neo Tri Beam in 3 has two different versions: if done correctly, it drains very little from Tien, but if done incorrectly, he replays the end of the Cell battle and collapses, most of his health gone.
Dragon Ball Z: Hyper Dimension differs from other DBZ fighting games in that instead of having a Ki meter, special attacks drain the player's health. Conversely, charging up can heal you.
Somewhat related: in the manga version of Prétear the Leafe Knights' Elemental Powers are connected to their own Leafe (Life Energy); overusing these powers drains their Leafe, regardless whether the attacks are performed by the Knights themselves or by Himeno when she merges with them. In the backstory, three of them died from overusing their powers in order to seal Takako, and were reborn; Shin, the youngest of them, dies for the second time after Himeno merges with him.
In the anime, to seal the Tree of Despair, Himeno overloads it with her own Leafe and dies in the process. Only Hayate's True Love's Kiss can bring her back
In the game of Monster World (a made up tabletop RPG) in Yu-Gi-Oh!, Bakura (a white magician) converted his hit points to magic points to keep up a magic barrier when Zorc was blasting them.
This is also the main rule in the Ancient Egyptian precursor to the Magic & Wizards/Duel Monsters game; Monster Spirits are summoned by sacrificing Ba (life force) instead of the modern life points system. Damage is also taken by decreasing the life force of the duelist, implying that a defeated duelist dies. See the "Card Games" section below for use of this tactic during the card game duels.
Dark Bakura's second tabletop RPG (Memory World, the Shadow RPG), which is based on the Ancient Egyptian Kingdom that played the above game (as well as the events), employs this for all the character cards. When a character uses his/her Ba to empower/summon a Monster Spirit or gets damaged, it decreases their Hit Points.
Yusuke Urameshi of Yu Yu Hakusho funnels his life energy into a last-ditch assault on at least one occasion. The strain leaves him unconscious and on the verge of (another) death.
Kuwabara does the same.
Kurama attempts a Heroic Sacrifice during the dark tournament using this trope. When he (just barely) survives, he discovers the magic fruit he's been using to temporarily become Yoko Kurama has been wearing off faster because Yoko Kurama's power is bleeding into his own.
This is how the original Shuffle Alliance defeat Devil Gundam Form II in G Gundam, at the cost of their own lives. The main characters later learn the same technique, but manage to pull it off without dying, possibly by virtue of being much younger and healthier than their aging predecessors.
Natsume Yuujinchou - Whenever Natsume frees a Youkai from his late grandmother's (and his own) servitude, it has a physically draining effect.
Sakura in Cardcaptor Sakura is physically drained by the effort of converting a Clow Card into a Sakura Card. When she learns that the cards will die unless they are converted, she attempts to convert six of them all at one time, half-killing herself in the process.
Some spells in Slayers can be so powerful that they draw upon the user's life force when cast- the best example is the Incomplete Giga Slave, which temporarily bleaches Lina Inverse's hair white after she casts it. The novels explicitly state that one of the defining attributes of a spellcaster is a high amount of stamina, as casting spells physically drains a person. When the setting was adapted as a Role-Playing Game, firstly under the Big Eyes, Small Mouth umbrella and then under the D20 umbrella, casting spells would cost health.
In the RPG, it normally costs nonlethal hit points (fatigue), but taking lethal drain is also an option.
In Fushigi Yuugi, Mitsukake can only use his Healing Hands once a day because of this. When he overdoes it towards the end, he dies.
Shakugan no Shana has this in Yuji. As a Torch, he's technically already dead, and, under ordinary circumstances, would inevitably be doomed to burn out and fade from existence. However, he's also a Mystes, and happens to have the artifact Reiji Maigo sealed within him, which replenishes his power of existence every midnight. As such, as long as he doesn't use up all of his existence in a day, he can lend his power to Shana, and, later, cast his own unrestricted spells, using his very existence.
One occasion in the manga being so severe, Sailor Moon's entire body disintegrated completely, she get better of course (because of the Galaxy Cauldron and the 'rebirth wave' she initiated), but damn.
Magical Girl Lyrical Nanoha gives one of these to Nanoha in the third season. It's called the Blaster System and it boosts Nanoha's power and gives her a set of Attack Drones that can cast her spells independently of her, for a potential boost of better than 4x her already monstrous power level. The cost of this is placing an immense strain on Nanoha's body and dealing physical damage to herself and to Raising Heart; when she used Blaster 3 in the climax of the third season, she lost 8% of her total magical power and had to spend the next few years on enforced vacation to recover it.
Sumomomo Momomo in the final few episodes, sort of. Koushi and Momoko were both poisoned, but Momoko was poisoned later than Koushi. However, due to her fighting it has sped up the poison and Momoko passes out due to the effects before Koushi does.
In Mahoromatic, Mahoro's most powerful weapon (usually manifesting as a plasma-like flame on her fist) is directly powered by her life force. Since her energy is running low to begin with, using it shortens her remaining lifespan dramatically. Mercilessly made explicit by the "Days until Mahoro stops functioning" counter that is shown after every episode. There will always be a significant drop in numbers whenever she uses it.
Infinite Stratos does a technological example of this. IS battles end when one side's energy shields are depleted in order to avoid injury to the pilot - in fact, the unshielded IS instantly shuts down to prevent further fighting. The protagonist's most effective (and for a long while, only) attack is Reiryaku Byakuya which saps his own shields to pierce through the enemy's and score an instant kill. The first time he used it, he had no idea how it worked and therefore lost the match because his shield zeroed out less than a second before the attack connected. After learning about it, he's understandably reluctant to use it against a manned IS ever again.
In fact, it was his sister who invented the attack and used it to win the IS world tournament.
In Mawaru-Penguindrum, Momoka Oginome claims to be able to do this. She says that she can change the fate of living beings via her Destiny Diary, but adds that she'll have to pay a price: suffering bodily harm in exchange for what she wants to do/fix/etc.. To save a bunny from dying, she casted a "fate changing spell" in the Diary and accepted to get a cut on her hand in exchange; later, to rescue her friend Yuri from her abusive father, she casted another one and got severe burns that landed her in the hospital. In fact, when Yuri tried to touch the Destiny Diary, Momoka stopped her from doing so to avoid a possible backlash from hitting her.
Toriko's Autophagy. If he runs out of energy and needs more, his Gourmet Cells "eat" his own body to gain more power. If the Autophagy goes unchecked, Toriko's body will eat itself to death.
In Pokémon Special, though it's never explicitly stated, it's safe to assume Yellow's powers fall on the mild side of this trope as overusing her powers runs the risk of exhausting her and putting her into a deep sleep.
In Naruto, jutsus require chakra and stamina to be used. In most cases, ninja get to the point where they're unable to use jutsu when they're low on chakra, but if they run out, they die. Except in Naruto's case, where running out of his normal chakra results in him tapping into the Kyūbi's chakra.
Rock Lee's Eight Gates are a more straight example of this. The more he opens, the more powerful he becomes, but the greater strain it puts on his body. It's stated that if anyone opens all eight gates (known as Hachimon Tonkō no Jin, or Eight Gates Released Formation (Eight Inner Gates Formation in the dub)) he or she will die, which is why the eighth and final gate is called Shimon - the Gate of Death. In Lee's case, his muscles actually ripped when he opened the fourth gate, appropriately named Shōmon - the Gate of Pain.
In Magi - Labyrinth of Magic, using too much of your magoi is dangerous because of this. Magi, who can use the magoi and rukh outside of their bodies, aren't too bothered by this.
In Death Note, a user of the note can gain the Eyes of a Shinigami, which allow them to see people's name and remaining life. The price is half of the users remaining lifespan.
While it's not strictly magic, One Piece has the Impact Dial, which can absorb and fire any physical force. However, the recoil is exactly the same force as the actual impact, meaning that whoever uses it takes as much damage as they deal with it. The improved version, the Reject Dial, actually increases the power of the impact, but it's said that using it twice will cause the user to die. After being told this, then proceeded to use it three times and survived.
Luffy's Gear Second leaves him thoroughly exhausted after use, as it speeds up his blood flow to increase his power, which also speeds up how fast his body absorbs and processes nutrients. Lucci even pointed out that there was a danger of dramatically reducing his lifespan by using it.
Necropotence is the card that truly emphasizes the usefulness of this trope; when it was released, its use dominated tournament play. Remember, tropers: the only truly important hit point you have is the last one.
The New Phyrexia set introduces "Phyrexian mana" (the symbol for which looks a bit like phi ɸ), which can be paid with either one mana of the appropriate colour or 2 life.
For the most part, whenever a card effect in the Yu Gi Oh card game requires a cost to activate, it's generally one of two things: a discard from your hand or deck or a payment of life points. Considering that the loss of either resource in their entirety means game over for you, this is very much a Cast from Hit Points scenario.
The third is typically the sacrifice of a monster, which also fits, even if it's not your hit points you're using.
The Lightsworn-archetype Deck uses the first type, as several of the monsters and spell/trap cards discard two to three cards from the top of your deck after activation or at the end of every turn. The recently added Psychic-type monsters normally drain Life Points to use their effects, but there are also a few that give some back.
In the under-advertised game Magi-Nation, ALL spells and abilities were cast from hitpoints. There was no MP or Mana to speak of, so monsters and your own character would use the same life force to cast magic with that they'd use to absorb damage from the enemy. Additionally, summoning your Mons cost the protagonist life energy equal to the beastie's hitpoints - in the video game its remaining HP would be refunded to the hero at the end of the battle. All this combined made for an interesting level of strategy wherein you would have to decide whether the loss of life was worth being able to kill the enemy that much faster (and also made heal spells rather dubious in their usefulness - the amount healed is almost always lower than what it costs to cast in the first place).
Shadow Era also has several cards that can damage the user. Some items (such as Rusty Sword) damage the user when destroyed, while others can constantly drain from your health for some benefit (like Enraged which allows the player to draw an extra card at the cost of one health a turn).
In Lycee TCG, since the orthodox way to lose the game is having no cards in your deck when you're supposed to draw one, your deck effectively acts as your HP. The more powerful Standard Abilities usually requires you to discard cards directly from your deck.
Malphast pulls what may have been a subtle use of this in PS238 (it's possible the loss was a side effect, rather than fuel), though it's not his hit points he uses to cure Tyler's sleepiness.
Malphast: I can't explain it in words you could understand, but rest assured your soul will recover in time.
Tyler: My what?
Thor's most powerful attack, the God Blast, channels his life energy through Mjolnir, combining their power for an attack that can drive away a hungry Galactus.
...the magic flowed obediently out of him and helped Bellatrix to her feet. (For it wasn't his magic he had expended, it had never been his magic that fueled the Patronus Charm.)
Innocence illustrates the deadly consequences of putting all of your life into the charm.
In The Covenant, the teenage characters find they can cast powerful magic, including flight (including the car they're in) and indestructibility, but each time they cast a spell, they age themselves. That last clause doesn't really take into effect until they turn 18 though. Technically, they can use all the magic they want to before then with no ill effects, but magic is addicting, and such behavior gives way to very bad habits. Meaning, if one abuses magic before they turn 18 and get full power, then they will most likely age themselves horribly very fast.
In Eragon, it is implied that magic exists but it is very rarely used because it harms the caster. Indeed, the main character almost dies trying to use magic.
The Young Wizards series has this as a common technique. Everything has to draw energy from somewhere, even magic, and the wizard's own energy sometimes represents the most convenient source. With everyday magic, this simply leads to fatigue if overused, the magic equivalent of exercising strenuously. Magic that can save the day, however, is often Cast From Lifespan instead. This arises several times in the series, including as the primary plot of the second book, Deep Wizardry. One shield spell in High Wizardry costs a year of the casters life for each blast it absorbs (granted, these are attacks from a distracted Lone Power), and the characters discuss it during the fight: "What if you're scheduled to get hit by a car or something in less than a year?" "I'd better look both ways then."
This isn't nearly as bad as it sounds, as dead wizards in all cases go to heaven, and living ones can visit whenever they like. Still, wizards live short lives; the girl casting that spell was fifteen, and has already survived a number of normally fatal situations, up to and including offering herself as a human sacrifice by accident - and then deciding it was worth it.
In the Malazan Book Of The Fallen series, the use of healing magic in great quantities is fatal for at least one supporting medic character.
In the Inheritance Cycle magic is cast by expending the caster's physical energy. It takes as much energy to do something magically as it would to do something physically. In addition, once an incantation has been uttered, the caster must commit to the spell, even if it kills them. Knowing your limits is very important for a spellcaster in this universe.
Of course, dragon riders have an advantage: a rider can borrow his dragon's hit points to cast spells. Dragons, needless to say, have lots. As of book 2, Eragon learns to cast spells by drawing energy from his environment, which kills the surrounding wildlife, but doesn't cause him any serious harm (it effects him emotionally though). In book 3, yet another power source is introduced: magical stones that come from dragons and also serve as their Soul Jars. In Inheritance, it is mentioned that One rider essentially turned herself into a matter/energy explosion during the Fall, rendering Vroengard a radioactive wasteland and killing at least one of the Forsworn in an extreme example.
Mages in Mercedes Lackey's Heralds Of Valdemar series can do this; the eventual result is known as "drain shock", which is usually fatal. Alternatively, a mage can simply burn themselves out like a candle to perform a "final strike", the idea being that if you have to die, by god you're going to take someone out with you.
Perhaps unsurprisingly, more than one protagonist mage uses the Final Strike to achieve a Dying Moment of Awesome .
If the Shin'a'in get absolutely desperate, one of their Shamans or Swordsworn may be called upon to make the ultimate sacrifice in order to invoke divine intervention.
Its implied that the same rules also apply for those with particularly power versions of the Gifts that also operate in the series. The power Lavan Firestorm unleashed is uncannily similar to a mage's Final Strike and the results equally cataclysmic.
The magic in Katherine Kurtz's Deryni works this way; using Deryni powers requires concentration and is physically fatiguing. The more powerful the working, the more exhausting it is, and repeated and/or extensive use of the powers in a short timeframe can cause a Deryni to blackout.
In Pocket In The Sea there is a rare non-magical example when Clase(Mother) uses her body as a buffer against the electronic feedback from a submarine mecha's poor-quality piloting interface. It almost kills her and is written in such a way to suggest doing so also shears years off of her life. In the same book it's also hinted to the reader that using psionic abilities also works like this.
In the Sword of Truth series, a wizard can cast Wizard's Life Fire, a powerful explosion that kills the wizard but usually reduces whoever is nearby to ashy stains on the walls. The taste of said ashes yields a clue as to why the dying wizard chose to cast Wizard's Life Fire: if the ashes are bitter, the wizard cast the spell to save himself from torture; if they are sweet, the wizard gave his life to save another.
In The Dresden Files, a wizard's "Death Curse" instantly kills the caster in order to inflict some horrible punishment on its recipient. Regular magic can hurt a wizard as well; when Harry conjures a rage-fueled firestorm in one book, he has a heart attack after sustaining it for too long.
This is more of a Taking You with Me as the death curse doesn't kill the wizard, it's what wizards do when someone kills them.
It may be both- there is a passage in one of the more recent books which actually describes the 'mechanism' behind Death Curses which states that the reason a Death Curse is so powerful is that it uses all the energy in the wizard's body, including that keeping him alive. Survival instinct is the usual reason that wizards don't cast it unless they're dying anyway, and unless they're being murdered they don't really have a motive to curse someone that they could take care of in some other fashion. If a wizard is sufficiently angry and no longer cares if he survives, he can cast his Death Curse anyway, and it will kill them. Harry almost does this before being snapped out by another character.
There's also the use of Soulfire, which allows a caster to infuse some of their own soul into their spells to boost the power and effect of the spell. Unlike Hellfire, however, Soulfire isn't destructive, but rather constructive. Harry ends up using Soulfire to generate a powerful hand-like construct of force to beat the hell out of a Denarian spellcaster. The drawback behind using Soulfire, of course, is that it uses your soul as the fuel to empower your spells. Partially drained souls in The Dresden Files universe do regenerate, and pretty quickly if you do soul-affirming things— but as Bob explains it very succinctly, if you subtract five from five...
Also, for ghosts, just about any form of attack besides Good Old Fisticuffs is one of these. This becomes a major plot point in Ghost Story when an important character becomes one.
In The Wheel of Time, channeling results in physical and mental fatigue, depending on the amount and duration of the channeling. In extreme examples, channelling have "pushed" themselves past usual levels, but it puts them at the risk of losing the ability to channel, or, in extreme cases, death.
Some of the Fighting Fantasy books, especially the aptly named Sorcery four-parter, have EVERY spell being cast at a cost of health.
In books by Tamora Pierce, desperate bad-guy mages often kill themselves by using their own life energy for magic once they've run out of any other kind of magic. Usually, this is accompanied by one of the major characters shouting at them to stop or else they'll kill themselves, a warning they never heed.
Although it's definitely not limited to the bad guys, as Ochobou burned out her magic and herself taking down five mages of her level in Trickster. And in the Circle book The Will Of The Empress, when the Discipline four (who at this point in the books are basically the strongest mages in the world when they stand together) use their magic to break the border of Namorn, they are left extremely hungry and tired for days. Briar says that if they hadn't drawn power from all across the empire, they would be dead.
The Name of the Wind uses a system similar to this - Sympathy is essentially a magical form of energy transfer. If you are good at it, and don't care about your own health, you can transfer the heat of your blood into something to set it on fire. This is not good for you.
One upside is that instead of running out of mana, sympathists run out of fire.
Beyond this, you can "run" a spell in your head as long as you don't mind some minor Eldritch Abomination taking a small bite out of your brain. Doing this too often, even accidentally, results in Kranzberg's syndrome and a permanent trip to St. Hilda's
How the Returned work in Warbreaker. Most people in that world have an energy called Breath that can be used to fuel magic, but if it's completely drained they just lose magical ability (and a certain degree of keenness of the senses) until they can aquire more from someone else. The Returned, however, are kept alive by one immensely powerful Breath- this allows them to perform miracles beyond the capacity of ordinary magic, for the cost of their life. The sword Nightblood, resident Artifact of Doom, also functions like this, drawing on the Breaths of its wielder to fuel its powers. If the wielder runs out of Breaths while still using the sword, the results... aren't pretty.
Sorta in Discworld. For a Wizard to do something, it takes as much energy to do something magically as it does physically, unless you can harness an outside force. Having no outside force makes the Wizard rely on the leverage of his mind, meaning if they try to do something too difficult, their brain flicks out their ears. Example: Galder Weatherwax makes a protrusion of stone on the University fall, allowing him to zoom upwards.
Psionics in The Second Gate normally channel energy they've "metabolized" and stored, but in a pinch, they can draw power directly from their biological functions. The mind instinctively tries to cut off psionic connections to prevent permanent damage at the same time, which can result in anything from mild burnout to a coma - which is usually too late to save the user anyway.
While after the first book of the Dragon Lance series magic functions by drawing power from the Moon Gods, before they are unsealed all magic is used with the caster's own energy, as shown whenever Raistlin uses too many consecutive spells and is left exhausted.
Magic-users in The Soprano Sorceress and its sequels use their own body's reserves to cast; they have to eat like bargees just to keep their weight up.
In Everworld magic-users are shown to weaken if they use too much power, leading Jalil to wonder if magic burns calories. Merlin, for example, is so tired after his battle with Loki that it's months before he is able to fully recover.
In The Bartimaeus Trilogy, the most vital part to a Golem's functioning is a magical scroll inserted into the construct's mouth. The writing of said scroll drains the vitality out of the writer to be transferred into the golem, and writing more than one manuscript (at least in a short time) is likely to kill you. (Though since golems completely neutralize all demon magic in their vicinity and all of magicians' power is based on demon magic, one is often enough.)
Live Action TV
An unusual example in Smallville: It is possible to grant temporary kryptonian powers to humans, but it has negative effects like putting Jeremiah in a permanent coma and giving Jonathan lethal heart problems.
The Face of Boe makes a Heroic Sacrifice in "Gridlock," giving up the last of his life energy to help save the thousands of people trapped in New New York's underground traffic system.
The Master, after Lucy disrupts his revival during "The End Of Time." He wins the Superpower Lottery as a result, but all his new abilities are fueled by his own life-force.
In "Rise of the Cybermen," the Doctor uses ten years of his life to recharge a power source in the TARDIS. Subverted in that ten years to a Time Lord is a scant few moments, and the Doctor's regenerations never last for a full natural lifespan anyway.
Witch's spells appear to take a severe toll on the body in The Vampire Diaries. To the point where it sometimes seems that Bonnie can't do anything useful (except mind-whammy Damon) without knocking herself out.
Most spells in Shadowrun have a chance of causing Drain, which usually deals the caster mental damage (i.e. fatigue). However, if one casts a spell with a power greater than his own Magic attribute, the Drain will cause physical damage instead. The rulebook describes this as causing nosebleeds, bruises, spontaneous wounding, and, in extreme cases, death by aneurysm, stroke, or the like.
Also worth noting: characters with mechanical or cybernetic enhancements have decreased Essence, which makes them bad at magic.
Mages in Mage The Awakening can burn some of their health for a quick boost in Mana. It also works the other way around, though this is easily the least efficient means of magical healing in the whole game.
In the previous game, Mage The Ascension, a substance called Quintessence makes casting spells easier. A mage that runs out of Quintessence can rip some from their own body, damaging it in the process.
Demons in Demon: The Fallen can enhance their powers by drawing energy from hitpoints... except that said hitpoints belong to their followers, not them.
In Warhammer Fantasy Ogre Butchers can cast a variety of Gut Magic. Along with the normal requirements of spells, they sometimes require the Butcher to inflict bodily harm on themselves. In particular the Trollguts spell, which is the best out of the 6 available to the Ogres, but permanently takes off one health from the caster that cannot be regenerated in any way (whereas the other ones are usually avoidable unless you displease the Random Number God, and can be regenerated with another spell).
The psionics in AD&D used a spell-point system even when the actual spellcasters use Vancian Magic. Since an ability like Cast from Hit Points fits in so much better with a spell-point system, the 2nd edition had "Cannibalize" power that allowed mid-level psionicists to get extra power points from damaging Constitution. 'Death Field' power causes to everyone in the area of effect damage proportional to the sacrificed Hit Points 1:2 (or 1:1 for users of evil alignments).
3E supplement Epic Level Handbook several extremely powerful spells, such as Hellball and Let Go of Me, work this way. The greatest example of this, however, is Vengeful Gaze of God, which deals 305d6 damage to an opponent while dealing 200d6 damage to the caster, who suffers from bleeding eyes and convulsing skin and, most of the time, dies. This spell will almost always kill anyone and anything it is used against, excluding the most powerful of monsters, who simply might be killed by it.
3E Fiendish Codex II offers the Hellfire Warlock, which upgrades the warlock's standard attack from "kinda okay" to "nuclear inferno" at the cost of 1 Constitution drain per shot. Since Constitution affects both current and maximum HP, it's generally a good idea to have someone on standby with a restoration spell or a cheap wand of lesser restoration with the spell provided by a Paladin (it is even suggested in the fluff).
To make it even better a Hellfire Warlock with one level of Binder can gain an ability that automatically heals 1 point of ability damage a turn. Then there's classes like Legacy Champion which increase your effective level in another class, even beyond the Cap. Combine the two and you get a supercharged Hellfire Blast usable at will.
The Blood Magus class from 3E's Tome & Blood can sacrifice a little blood (hit points) to cast spell with slightly harder saving throw or replace material components.
3.5 Complete Arcane replaced it hit points damage with Constitution.
And the 4e Blood Mage paragon path allows you to take damage to deal as much extra with encounter and daily spells. This was so abusable it needed to be nerfed with errata. Twice.
Forgotten Realms of AD&D era had some of more formidable spells involving sacrifice of the caster's hit points — either normal damage, permanent, or the loss incurable as long as the spell is active. This includes several spells from Secrets of the Magister. Which may be a legacy of old Elven Blood Magic, which includes 'Blood Dragon' — near-unstoppable mass killing spell requiring the caster's death. Also, the Drow sometimes have "body weapon" enchantments as a last-ditch defence, which usually involves loss of a body part or other physical injury. E.g. Jalynfein by breaking a finger and saying a word could fire a burst of 24 Magic Missiles (cast normally, would be limited to 5). Phaerimm dehydration spell 'lifedrain' (the one which made Anauroch a desert) also involves permanent sacrifice of a hit point, but holds for years — and dies with the caster.
Spelljammer spell "Create Atmosphere" involves permanent hit point sacrifice from the caster. It makes a cubic mile/level of the air self-renewing for more than a year, after all.
The 4th Edition Bloodclaw Weapon would let you pay a small amount of HP with every attack, which then would be doubled or tripled if the attack hit. This ended up being so much more powerful than other weapon enchantments (especially for Fighters and Barbarians, which get more HP than other classes) that it was nerfed to a once-per-battle use and it STILL managed to be usable.
The Blackguard subclass of the Paladin from Heroes of Shadow uses a variant of this mechanic as well, which is powerful enough to be their entire Striker damage bonus.
The previous edition (using the Revised Core Rulebook) of the Star Wars RPG rules generally had Force powers cost vitality (the system's version of Hit Points) to activate. If you didn't have enough vitality, you could even use wound points (representing real and dangerous—even potentially fatal—damage) to make up the difference. The only thing stopping characters from 'casting to death' is the fact that no Force power had a vitality cost so high that the damage could push a character far enough into the negatives to result in death.
In the cooperative play game Middle-Earth Quest, your hero deck is also your 'life pool.' Any card you play in combat, or even to move around the map, costs you a hitpoint.
In the German tabletop RPG The Dark Eye (aka Das Schwarze Auge in German), every magic user can do this, but not without consequences, usually additional damage and that damage might permanently reduce the maximum hitpoints of that character (only when he drops too low as a result of blood magic though). Excessive use of this in one of the novels leads to a mage permanently losing his ability to use magic. Later on, he uses a magic sword that also drinks from his Life Energy, losing fingers on his good hand as a result.
Epideromancers in the tabletop RPG Unknown Armies power all their magic by hurting themselves.
In the live-roleplay system Labyrinthe, almost all supernatural abilities have an hp cost in addition to a mana cost. The amount of damage done is relative to the level of the ability relative to the level of the caster.
In the Swedish Tabletop RPG Chronopia, Orcs have access to a very interesting magical discipline; Painmagic, ripping off a finger can grant you skill bonuses, cutting yourself can give you visions of the future and hacking off an arm or a leg can make you temporarily invulnerable. Not surprisingly, they have also developed plenty of rusty prosthetics complete with hidden sawblades and other nasty surprises to replace those limbs lost.
GURPS allows casters to do this, though it's more difficult than using other energy sources, presumably because the pain makes it hard to concentrate. Usually, spells are powered with Fatigue Points (i.e. wizards get tired when they cast spells) or with enchanted "energy batteries" called powerstones. Once you burn through all your available FP (or earlier, if you choose), if you keep casting spells without resting, you start burning HP. Ordinarily, you can only use up hit points until you lose consciousness, at which point the energy drain stops (you don't die).
The supplemental advantage "Word of Power" drains so much fatigue that it's guaranteed to drain life from a normal person. It will keep speaking itself even if the caster dies in mid sentence.
Pokemon Tabletop Adventures has the Psychic class, capable of using certain pokemon attacks, similar to the Martial Artist class. The martial artist's attacks can only be used a certain number of times per day, whereas the psychic's attacks can be used at will, but require this trope. (Thankfully, the nature of the psychic's key stats means they usually will have a large amount of HP to cast from.)
As mentioned above, casting or controlling a spell in Slayers d20 is based on stamina (a Fortitude saving throw modified by caster level), and deals subdual damage to the caster based on the spell's difficulty and your margin of success. You get a hefty bonus to your control checks by voluntarily taking lethal damage, or it might happen anyway if you botch horribly enough.
Champions characters who run out of Endurance can continue to use their powers by taking Stun damage, at a rate of 1d6 Stun per 2 Endurance required. This only works for powers that draw on the user's own Endurance pool, as opposed to the Endurance Reserve power. A character can literally knock himself out from overexertion.
Ixtli, the Aztanli-specific Boons in Scion, have a number of abilities that grant extra Legend for physical sacrifices. The amount gained from bleeding another creature is half what you get for doing the same thing to yourself.
Several powerful Charms and spells in Exalted require you to sacrifice health levels as part of their activation cost.
In high-paranoia games, where every attack might kill and so every attack must be answered with a Perfect Defense, every attack is cast from hit points: they cost charm activations and Essence, the two resources that fuel perfect defenses.
There are also two spells that can be cost for minimum Essence (mana, magic points) expenditure but automatically kill you and deal significant damage to everyone around you.
In fact, Dragon-Blooded have quite a percentage of Charms with the "Martyr" keyword. That means that they can be cast with greater effect, but killing the Exalt for sure. That is why they can be used with no Essence left. And some of such Martyr usages can last for generations.
Fireborn had this as a potential side effect. It takes one(or a group) so much power to cast a spell to be built up. Depending on the situation, one can roll a lot of dice and hope to quickly cast it, or do so slowly and carefully. However, if you go over the needed number of successful rolls and charge up too much power, the excess physically damages you. Of course, one can eliminate this by learning ways to channel that overload into the spell, usually for enhanced range/duration/effect.
The Sorcery power in the 1980's DC Heroes game had a function similar to this. Every time the power is used, the AP's (power rank) used is compared against his or her Spirit score (a combination of damage resistance and hit points versus mystical damage). Effectively, if he or she is using AP's lower than his or her Spirit, there's no problem. Otherwise, there's a chance of Spirit damage (affects the "hit points" versus mystic things, but not durability, that's always your maximum Spirit). If there's a significant difference between the two with the Sorcery being higher, the caster will likely be rendered unconscious by using a full powered spell.
In Sentinels Of The Multiverse the hero Nightmist has multiple spells that damage her as well as achieve whatever the intended effect was.
In DOTA Allstars, a custom game based on Warcraft III, Sacred Warrior (Huskarr)'s Burning Spear ability is entirely Cast from Hit Points. Lord of Avernus (Abbadon)'s Death Coil skill and Phoenix (Icarus)'s Sun Ray costs both mana and hitpoints.
In Radiant Historia, one of the nodes requires you to decide whether or not Aht will do this to help win a fight so that you can get info to find Eruca and the rest of your party.
The Paladin's "Martyr's Reckoning" in Ragnarok Online can take away 9% HP for a burst of damage.
Ancient Domains of Mystery allows this as a standard game mechanic for spellcasting: if you don't have enough Magic Points to cast a spell, you can "exhaust yourself" casting it, expending Hit Points to pay the remaining cost. The cost may be fatal, which is to be expected in this game.
Amusingly, however, a sufficiently skilled caster can pay for a healing spell from hit points and heal more damage than he just took. In addition, some older versions allowed casting spells from books and still have a greater healing rate than HP loss, without any other cost.
This also "abuses" the Mana stat, however, causing it to rise more slowly (or even drop) if done too often. It is one of many things in the game that grant an immediate benefit for a long-term cost.
In Final Fantasy IV, Tellah casts Meteo(r) to destroy Golbez, at the cost of his life. Unfortunately, it doesn't work.
Specifically, Tellah has 90MP max, while Meteor requires 99MP. Add to this the three heavy spells he casts just before (using a total of 110MP), and you have one dead mage.
Dark Knight Cecil's Dark Wave ability from the original version and the GBA remake does damage to all enemies at a cost of some of Cecil's HP. The DS remake version increases melee damage, but costs HP every attack.
This was also done earlier in Final Fantasy II, in which Minh (or Ming-Wu, or whatever he was called in the version you played) spent his life force to break the seal on Ultima.
Next, in Final Fantasy V Galuf uses an unprecedented amount of Heroic Willpower to continue fighting at zero HP throughout an entire boss fight. That may not seem like this trope, until the effects are shown afterwards. Galuf drained himself to such a degree that healing items and ressurection spells have no effect on him.
In Final Fantasy VI, the skill 'Pep Up' gives the caster's HP and MP to another character and then removes the caster from battle.
In Paladin's Quest, all spells draw from HP. Casting a healing spell kills the caster and heals the rest of the party.
NPC: MP? There is no such thing as MP on Lennus. We only have HP. You sound like an idiot! Where did you hear such superstitious nonsense? Your own HP are used to cast magic.
Becomes an aversion of Gameplay and Story Segregation in Persona 3: The main character's Eleventh Hour Superpower to seal away Death at the end of the game uses up all of his hit points. He hangs onto life for about a month after the final battle, apparently running on pure willpower, before slipping into a coma and dying.
There's also the Recarmdra spell, which fully heals/revives all other allies in exchange for setting the caster's HP To One or outright killing them.
Final Fantasy XI has a spin on this. Red Mages can't directly use HP for spells, but they can use the Job Ability "Convert" to swap their HP and MP, effectively restoring their magic with their life force. The ability can't kill you, however. If you use Convert with 0 MP, it just does nothing.
More recently, the Sublimation ability was added to FFXI, which allows a Scholar (or a very high-level mage with a Scholar subjob) to bleed HP into a status effect for later use as a sudden boost to their MP. Sort of a much slower version of Convert.
There are two White Mage merit job abilities like this: Martyr, which lets you sacrifice some HP to heal an ally by a larger amount of HP (useless), and Devotion, which lets you sacrifice some HP to refill some of an ally's MP (useful).
Final Fantasy VII has a materia that works similarly, switching the values for your total HP with that of your total MP, so you would have a character that has only a few hundred HP, but thousands of MP.
Final Fantasy Tactics has Worker 8. Worker 8, while being an Atheist (0 faith, the stat used when determining the effects of magic), can still do abilities without MP. All it takes is some hit points...
Also the Squire ability "Wish" allows you to heal an adjacent ally by giving him some of your hit points (you actually give twice as much as you lose)
In Final Fantasy Tactics Advance, Backdraft does a portion of its damage to the user as fire damage, but the user can avoid this with something that enables them to resist, be immune to, or absorb it.
In Final Fantasy Tactics A 2, units start with 0 MP and generate 10 MP each turn, meaning that double-casting or casting of high-cost spells can't be done immediately. As a result, Blood Price is available as an always-on S-Ability that lets the user pay for a skill in double its MP cost in HP. This ability can be abused quite easily by Red Mages, who can cast two spells in succession: one to attack, and one to heal themselves covering the cost of both. Interestingly, several enemy units that have Blood Price equipped can't possibly learn it through normal means.
Becomes rather meaningless with certain factors like casting Unicorn or Curaga with yourself in the range of the blood price. You wasted at most twice their MP cost and easily regain it back (unless you are absurdly low in terms of magic and even then, that is practically impossible).
Fighters also have Back Draft, a strong fire-based attack that slightly damages the user as well. However, they can equip fire-absorbing equipment, making it an attack that heals the user. Same applies for other similar moves but with different elements (the Lanista version is Dark).
One of the abilities Steiner can learn in Final Fantasy IX is "Darkside", an attack that does darkness elemental damage, but drains his HP.
Final Fantasy III, Final Fantasy IV, Final Fantasy VIII & Final Fantasy X-2 all had the ability Darkness (or Souleater), a spell that reduces your HP but not MP. Cecil could use it while he was still a Dark Knight, Diablos has it as one of the commands it can have and the Dark Knight dressphere. However, in X-2, it used no HP if you had the auto-ability Spellspring, which sets MP cost to 0.
Various spells throughout the series (starting with Final Fantasy VI's "Pep Up") sacrifice the caster to greatly replenish others in the party. Cue some WTF moments when the user is the only person in the party (Strago or Sabin in the Coliseum).
In the Lost Kingdoms games, you'll start using health for the cost of cards instead of magic stones. In the second game, you could cast yourself to death if you use too much health since the first game left you with some mercy health if you overcasted (and it fixed possible Game Breaking too).
In Dungeons & Dragons Online: Eberron Unlimited, one of the abilities gained by Paladin's, 'Divine Sacrifice', allows the paladin to sacrifice 5 HP to make an attack with a large damage boost. The HP are lost even if the attack misses, but in a setting where mid-level characters can have over a thousand HP, it's an insignificant price.
There is also an item allowing you to sacrifice 25 HP to regain 50 spell points, three times when fully charged (5 if using the Epic version)
Many classes in World of Warcraft have spells and abilities that convert health into that classes form of mana. There are also spells that force a target to do this, and spellcasters tend to have a lot less health than mana.
The Blast Mask in The Legend of Zelda: Majora's Mask provides Link with a limitless supply of bombs with the catch that each one causes damage. Raising your shield, however, will prevent the damage even though the source of the explosions is strapped to your face!
Sierra's Betrayal At Krondor and Betrayal at Antara drained stamina and hitpoints to cast spells, making it tricky to judge how much power to put into it. Characters' skills start to degrade once you use up stamina (either through magic or damage from enemies), and resting outside only recovers stamina up to a certain percentage. Even the one healing spell, Gift of Sung, only heals at a 1:1 caster to target ratio, so even when you have two spellcasters when you get Pug/Milamber in chapter 8 (and restore a small, small percentage of his magical ability), you can't heal everyone above the amount of health you had at the beginning of the battle.
Particularly problematic is the spell "Mad Gods' Rage": it calls down a series of lightning bolts at a cost of 3 stamina/health per bolt, until either the caster dies or all enemies are dead.
In Fire Emblem Gaiden, priests burned their own hit points to cast their heal spells. Fortunately, the priest's attack was a drain spell.
Mages also drained their HP when they used magic. Fortunately, male mages automatically healed themselves to make up for magic being their only attack, and female mages could attack with swords to conserve their health.
One of the Erk/Pent Support Conversations features Erk [storywise; the player need not do this] casting too many spells in a short period of time and passing out. Pent has to transfer some of his magic to Erk to save his life. The implication is that all magic-users "cast from hitpoints", all the time. Naturally, Gameplay and Story Segregation prevents this from affecting their actual HP.
Micaiah's "Sacrifice" ability in Radiant Dawn worked the same way, using up 1 HP for every 1 HP restored (and would use up as much as possible until the target was fully healed or she was at 1 HP.). Most players consider its only uses to be level grinding (it grants XP to Micaiah, who can then be healed to give XP to the healer as well), particularly after she class changes and can use a proper staff, and the fact that it heals stats for free.
The Sak and Nasak techniques from Phantasy Star II and Phantasy Star III work this way. They kill the user but fully restore any other living party member's HP. They require 1 TP to cast as well, though. The espers in Phantasy Star IV also have the ability to extend lives at the cost of their own lifespan. This is only plot-related, though, since the aforementioned techniques don't exist in-game.
In the not-so-renowned video game adaptation of Eragon, all characters have no MP whatsoever and use their HP to cast.Luckily, the "Magic" skill level of each can lower the amount of HP needed for magic.This renders healing spells slightly more useful (With low Magic skill they heal less than they cost) and is fundamental to avoid making the game Unwinnable once the player has to face, solo, a pair of bosses that are immune to pretty much everything except magic.
In Phantasy Star Online, there are weapons that sacrifice TP (technique points) for a special attack, some that drain money, as well as those that draw from your HP for a special attack. There's also a weapon that makes it so using techniques drains you HP rather than TP.
Castle Of The Winds forces the player to draw on their own constitution when they run out of magic, lowering their max HP in order to keep decreasing mana into the negatives until the player dies. Can turn into a bit of a nail-biter when you're forced to do so, as it has a nastily unpredictable habit of suddenly killing you. Also turns into a Good Bad Bug, though.
Star Ocean Till The End Of Time has a variant of this. You have separate HP and MP meters, but you are incapacitated if your HP or MP runs out, and some enemies deal MP damage instead of HP damage. Also, many of your special abilities, especially those belonging to nonmagical characters, cost HP instead of MP. Of course, MP in this game means Mental Points and the origin of the special abilities' power would decide what kind of fatigue the user would endure with some characters having both types.
You can't actually knock out any of the characters with this as the game simply doesn't let you do a move if you don't have enough HP or MP use it.
Enemies follow this rule as well and some enemies had massively low MP stats for their HP making an MP kill on them much easier than an HP kill.
Likewise, a few enemies (including Atma/Ultima Weapon) in Final Fantasy VI were classified as "magical creatures" and would die at 0MP. They couldn't actually kill themselves with their own magical attacks, but a single MP-draining attack from the heroes could end them.
City of Heroes has at least two powers like this; Oppressive Gloom, which keeps nearby enemies stunned while its toggled on by draining a small amount of HP, and Energy Transfer, one of the most devastating single-target melee attacks in the game, until it got nerfed recently. It's still devastating, but it now takes much longer to use.
There's also the healing power "Absorb Pain" and more recently "Share Pain." The first is a Hero only power that heals far more than any other healing power in the game. At the cost of a percent of the caster's health, and rendering them unable to recover HP in any way for several seconds. Share Pain is the Villain version of the power. It's almost identical to the Hero version in effect but rather than giving your health to the target, you take on their pain and getting a rush from it in the form of a boost to the damage you deal.
Knights of the Old Republic 2 has an interesting variation on this. There is a series of three powers called Force Body. The first enables you to split the cost of any other Force power 50/50 between your health and Mana Meter. The second and third split the cost 40/40 and then 30/30, actually reducing the cost of the power in question. Given 1: Force points regenerate fast 2:you need to blow a turn to use it 3:After the first few levels nothing really dents your force point pool enough to warrant it.
The Force Unleashed gives the character a rechargeable "force gauge" to measure how much force powers they can freely use. If you run out, points are reduced from hit points instead. This same system is used when the characters crossover into Soul Calibur IV.
Jedi Knight: Jedi Academy has a power called Force Rage. It drains your HP, but while it's active you're literally unkillable and you hit a lot harder. Problem is, you're weaker for a while after in addition to having lower health. At its lowest level, it's basically a means of Press X to Die, as the health drain and post-rage weakness are at their highest and the benefits are at their lowest.
Jedi Consulars and Sith Inquisitors can convert health into force in Star Wars: The Old Republic, but doing so also lowers force regeneration for a short period.
The spells Bone Spirit and Blood Star in the original Diablo drain both mana and hit points when cast. Their spell icons are red to signify this.
There's a curse that forces this on characters from Diablo II. Very nasty for Squishy Wizards, who often have less hit points than mana.
Specifically, the spell that does this causes one of two curses. If mana is greater than life, cast from HP curse shuts down the squishier players in short order. If life is greater than mana, however, it just causes a reduction to defense scores (the game's math is such that "defense" is rarely practical by the time this curse comes around, especially for the squishy ones). Since the effect is determined by the one creature it is cast on, rather than every creature in the effected area, teamwork tactics quickly reduce the threat this curse presents from "screwed" to "not good"
Those are two different curses. The first only works if you have more mana than life. Otherwise, you won't ever encounter it.
The Paladin skill Sacrifice in Diablo II does more damage than a regular attack at the cost of suffering 8% of the damage dealt.
Using the Rune of Punishment in Suikoden IV costs the user a chunk of HP.
Until you fix it, anyway... At which point it heals the hero while doing damage to his enemies. This makes him a Game Breaker in Suikoden Tactics.
The Rune of Condemnation in Suikoden V, being the child rune of the Rune of Punishment, is just like that. It can heal too depending on which spell you use.
Because his battery life is drained by doing anything ANYTHING, be it shooting a laser, scrubbing a stain, or just standing still, but it is also his life meter for whenever he's attacked, the titular character from Chibi Robo falls under this trope.
RuneScape has several Lunar spells that do this. Heal Other and Heal Group allow the caster to pay their own life points to restore an equal amount to allies.
Energy Transfer is an even better example, since it allows the caster to pay life points to give their own special attack energy energy to an ally (who is, presumably, better at fighting than the Lunar Mage, and could put that energy to better use). Heal Foo transfers life points on a one-to-one ratio; Energy Transfer actually consumes life points.
The Stimpack upgrade for the Terran Marines on Starcraft allows them a short-term boost of speed and reflexes. Side effects include insomnia, weight loss, mania/hypomania, seizures, paranoiac hallucinations, internal hemorrhaging, and cerebral deterioration, represented by a loss of hit points in-game. Of course, in the expansion they give you medics, letting you use multiple stimpacks while countering the side effects, making a squad of marines with a few medics a very formidable force.
The Zerg Defiler in Starcraft and Undead Lich in Warcraft III both have abilities that allow them to consume allied units to restore mana - allowing them to cast from other people's hit points.
Metroid Prime 3: Corruption does this for the use of Hyper Mode. When Samus goes into it, she instantly loses one tank of energy. She can gain back some of the lost energy by exiting Hyper Mode early, making it a surprisingly effective defense if you only turn it on when getting hit by a high-damage attack.
Parasite Eve 2 had a Berserk status ailment that follows this trope. When Aya is beserked, using her weapons will sap her HP. Spells require HP to be used instead of MP, but the casting cost is doubled. While Berserk increases Aya's weapon power and levels up her parasite powers by 1, the HP cost that follows may not be worth it.
Strangely warped in Rune Factory. Nearly every action costs RP, but if you have none left, it will drain HP instead. The only thing you can'tCast from Hit Points is, well, actual magic.
In F-Zero X and GX/AX, Nitro Boosts can be activated at any time after the first lap but drain your vehicle's life meter when used.
Inverted in The Elder Scrolls II Daggerfall. Your character has a chance of absorbing the magicka (mana) of the spell being slung at him and add it to his own reserves but, if you absorb much more magicka than you have capacity for, you die.
The Sigil in Strife is the most powerful weapon in the entire game. Its ammo is your hitpoints of course, and it uses more when you upgrade it by finding the other pieces of it.
Iji: If you manage to avoid killing anything in the entire game, a NPC will leave behind the Massacre to help you in the final boss fight. Each shot takes one Health to fire, but the damage dealt is only slightly less than what you'd deal with reflected shots.
While not specifically magic per se, the Warrior class from Maple Story gains skills that, in exchange for Attacking Multiple Enemy's in a Single Attack, required payment from both HP and MP — counterbalanced by the fact that these guys happen to have the highest HP stat in the game, hands down.
And in an inversion, the Mage class gains a defensive buff that allows them to redirect up to 80% of the damage they receive to their MP.
The Melee-based Brawler path of the Pirate class also learns a skill that converts HP into MP. The higher level the skill, the less HP you have to use to gain MP.
The Demon Avenger plays this trope straight, literally casting from his/her own hit points — because s/he does not have an MP meter!
In Aidyn Chronicles: The First Mage on the Nintendo 64, magic is cast using stamina, which along with endurance and character level determine total Hit Points.
After a Pokémon has used all of their PP for all of their moves or is under a certain condition (Like if holding a Choice Scarf and being subject to Torment), they are forced to use use a (otherwise unavailable) move called Struggle, doing a pittance of damage at the cost of a sizable chunk of HP.
There's also the actual move Curse, where a ghost Pokemon can sacrifice a large chunk of their HP to put an extremely damaging curse status effect on the opponent (oddly enough, it only has this effect for Ghost types, and by powerful, I mean it apparently requires the loss of HALF the users health to put a curse which takes away a QUARTER of the opponents health per turn). And has the illustration of a needle going into the ghost type. If any other type uses it, it just decreases speed, but increases attack and defense.
And Substitute, which takes 1/4 of your current HP and turns it into a, well, substitute that takes the damage of the next attack, as well as block status.
A hold item called Life Orb boosts attacking moves by a 30% at the cost of 10% of your Max HP. The abilities Magic Guard and Sheer Force (for moves that affect it) cancels this out though.
Some powerful moves such as Take Down, Wood Wammer, Flare Blitz, Brave Bird, Head Smash, Double-Edge, Volt Tackle, and Submission do recoil damage to the user. Some Pokemon, such as Aerodactyl and Geodude have the ability Rock Head, which prevents recoil damage. Others have the ability Reckless, which raises the damage and the recoil.
Jump Kick and Hi Jump Kick are powerful, but when it misses, the user gets recoil = 1/8 (1/2 in Black/White) of the damage it would've dealt.
Belly Drum costs half of your HP to maximize attack. It can be Baton Passed, thus making it a deadly strategy, especially if a Pokemon like Smeargle uses it.
Self-Destruct and Explosion (especially) are extremely powerful, but it instantly causes the user to faint.
Final Gambit causes the user to faint and then subtracts whatever amount of HP they just lost from the target.
Healing Wish and Lunar Dance cause the user to faint, and completely heal the health and status effects of the next Pokemon to come in.
Memento will cause the user to faint, and then sharply lower the attack and special attack stats of the opponent.
In the field, Softboiled can be used to restore someone else's HP, at the cost of that same amount of HP from the user.
Pokémon XD: Gale of Darkness has two moves of its own. Shadow Half halves the HP of everyone else on the field at the cost of half your HP. Shadow End is basically a shadow version of Double-Edge - it does massive damage and hits you with recoil.
Blaster System of Nanoha ver. StS in Magical Battle Arena. In Magical Girl Lyrical Nanoha, it enhanced Nanoha's capabilities, but ate at the life of both her and her device with every spell she used. This was translated in the game as a move that will significantly increase her damage at the cost of her HP being drained while it's active.
Dekans in Rohan Online have a number of powers that draw on their health. Forefoot Step is a melee attack that uses up 20% of the Dekan's health and deals out damage based on the amount of Hit Points sacrificed (and is best used with tank builds for both damage and survivability). Health To Mana sacrifices health in order to give you more mana for skills, and Health Funnel draws off some of your health in order to heal others or yourself if need be.
Michael Jacksons Moonwalker has a bunch of special moves that come with the cost of draining your life bar, including throwing your hat as a boomerang and causing all the bad guys on the screen to get their groove on, after which they all drop dead. They don't drain health in the Arcade Game, only the console versions.
In the Ecco The Dolphin games, the Oxygen Meter running out meant Ecco's health would start to go down. If that ran out, he would drown. Surfacing would restore the air meter, but he'd still have to find some fish to replenish any health lost from nearly drowning.
There are several such spells in Guild Wars almost all of which belong to Necromancers. Several Blood Magic spells and some Death Magic have a minimal Energy cost, but require you to sacrifice a percentage of your HP to use them. See this wiki page for a listing of such skills.
In Vagrant Story, Ashley can learn special weapons-specific, and usually rather powerful or otherwise damn good, attacks called Break Arts. Using Break Arts doesn't cost MP, and doesn't increase Ashley's RISK. It does, however, take off a bit of HP. How much HP is taken varies based on the "strength" of the Break Art.
In the original Blood Omen, Kain had a spell that allowed him to literally shoot his own blood at enemies. If it hit, he'd gain back any lost health plus some extra, as the enemies' blood would be sucked out of their bodies and into his. If it missed, or if he fired it at something with poisoned or tainted blood, he'd be SOL.
In the MMORPG Dark Ages by Nexon, the Monk class had access to four different forms of martial arts: Draco (Dragon), Kelberoth (a large cat-like creature), Scorpion, and White Bat. One of the abilities that came with Kelberoth Form was Kelberoth Strike, which dealt a large amount of damage to an enemy at the cost of a large chunk of the Monk's HP. As a result, Kelberoth Form was generally derided as inferior, suicidal, and utterly pointless for leveling purposes unless you had a Priest attached to your hip at all times.
In Angband, if you cast a spell without enough mana, there's a chance of damaging your CON (health stat) temporarily or permanently.
Also, the Genocide/Banishment spell will subtract 1d3 HP for each monster killed by the spell. Potions of *Healing* recommended when casting said spell in a monster pit.
Dark Age Of Camelot has a hand-to-hand class called the Savage that can buff themselves with faster and more damaging hits at the cost of losing a percentage of their current health once the effect ends.
Legend Entertainment's Shannara (which is a videogame based on the books) has this for elfstones. "Mostly, the elfstones cannot be used twice without rest, or the user will be drained to the point of death." The elfstones themselves are also destroyed if used twice in succession. Naturally, Davio has to die by using them twice in a battle near the end-game. (And earlier on, you can save Shella if you agree with Davio using the elfstones a second time, and Davio doesn't die, but does lose the elfstones, which instantly forfeits the game)
In the books some magic works like this too, in 'The Elfstones of Shannara' Allanon's magic ages him substantially. Luckily he can recharge, which also handily explains why he's never arround except when adventuring needs to be done.
Wild Arms 4 and Wild Arms 5 both have a move simply titled "Sacrifice" which uses a little MP, which brings self down to 1 HP, but deals damage proportional to it. Since all HP returns to max after battle, this can become a Game Breaker for random encounters.
Sacrifice returned in Wild ARMs XF, attached to the main character's class. Her armor also came with the natural ability to put her to half HP instead of 1 HP.
Also, the "Time" and "Rounds" are essentially the life force of the Wisp:Time will always go down whenever one of your units is charging an attack.
In Tsukihime, the Mystic Eyes of Death Perception cause brain damage and severe upon every use. They're also quite taxing the psyche. How drastic the damage is depends on what Shiki is trying to destroy, and merely severing alone lines does less still. The low ends of the scale is merely slicing or destroying a normally living creature. The farther end is potentially lethal and covers trying to kill objects, concepts and properties, such as walls, the senses (theorized but never utilized) and the toxic factor in poison.
In Dragon Age: Origins, the Blood Mage specialization takes this trope to the extreme. Instead of mana, a Blood Mage literally uses his own blood (as represented by HP) to cast spells. While you have Blood Magic active, healing effects restore much less of your health than they otherwise would, which makes Blood Magic rather dangerous to use in combat if you don't know what you're doing.
A Blood Mage can get around this by draining blood from his or her own allies to restore HP. Ranger summoned pets will also do in a pinch.
Reavers are also warriors that have similar abilities.
The Soldier's Peak DLC allows the player to unlock a few Grey Warden-specific abilities, which drain health in return for certain benefits. For example, the Dark Sustenance spell allows the Warden to sacrifice a small amount of health to gain a larger amount of mana. Warriors get an ability that weaponizes High Pressure Blood to blow enemies across the room in all directions.
In Archon, the Wizard and the Sorceress units have pools of magic that they can use to hinder their enemies. Actually using them, however, cuts into their maximum HP when they enter combat. This can be dangerous toward the end-game, when your spellcaster may be one of the few pieces left on the board and they'll need all of the health they can muster.
Descent has the Fusion Cannon, a Wave Motion Gun which drains your shield energy if you charge it too long (and it does less damage if you do so).
In League of Legends, Dr. Mundo, Mordekaiser and Sion use health to fuel all their abilities. Olaf also has one ability that's fueled by health instead of mana. Then there's Vladimir the Crimson Reaper, who's an actual spell-caster rather than a fighter like the others.
The Life Leech from Blood normally uses trapped souls as ammunition; if you run out of those, it uses your HP as ammo instead. It's really a non-issue, considering it drains health from any enemy it hits and adds it to your own.
Colette's Sacrifice spell from Tales Of Symphonia. Still costs an incredible amount of MP on top of it, though.
In Tactics Ogre The Knight Of Lodis, Euphare's Energy Transfer skill lets her give up 1/4 of her HP to restore another character's MP. Since all characters start each battle with 0 MP and recover a bit per turn, Euphare's skill borders on a Game Breaker, letting you use powerful summons and other spells immediately on turn 1.
In the Doctor Who spinoff game Destiny of the Doctors, the Graak starts with 9999 hit points, but any exertion reduces that. Fortunately, there are enough restorative crystals for it to gain enough hit points to finish restoring the Doctor's incarnations.
Additionally, if the item combined with a Pastamancer spell known as "Cannelloni Cocoon," which heals the user's full hit points for 20 MP, it wouldn't really matter how hard hitting death was. Full hit points for a fraction of the magic you just regained.
In Mega Man Battle Network 4 and 5, the use of a Dark Chip will allow Mega Man to perform a very powerful attack, at the cost of losing 1 hitpoint permanently for each Dark Chip he uses.
Not specifically magic, but in numerous Beat Em Ups, such as the Final Fight trilogy and Streets Of Rage 2 and 3, characters have special attacks that knock nearby enemies down, but have a health penalty if they connect.
In Arcanum: Of Steamworks & Magick Obscura, magic is kind of funny. There's no mana meter, per se, instead there's a blue "fatigue" meter which gets used up if you overdo physical actions, take blunt weapon hits, and is used up when you cast magic. An unwise spellcaster can cast themselves into unconsciousness rather quickly. So magic, in this game, doesn't kill you by casting it, but it does take a physical toll on you and if you overdo it you will wind up getting killed anyway by all the enemies piling onto your unconscious body.
Top-down MMO space shooter Subspace is all about this. Firing bullets, launching bombs, leaving mines, and using your afterburner all drain your shield energy, which regenerates slowly.
Although NetHack characters themselves can't cast from hit points, wands can — a wand with nominally zero charges still has a little magic left, and with repeated zaps, it will convert its mass into enough power for one more shot, after which it disintegrates.
The spinoff Slash'EM does allows casting from hit points if power point is low, and the character is at least skilled in the school of the spell in question.
Iari's Golemn spells in Summoner 2 use HP, and explosion knocks her out. If you've got the right gems equipped, it's virtually the only way to kill her late in the game, so make sure you have it if you take her into Survival in the Arena, or you'll be there forever.
In Birth by Sleep, one of Terra's unique attacks is "Sacrifice", which consumes HP - although never enough for you to KO yourself using it.
The ability Risky Play in 358/2 Days causes you to take damage every time your attack misses. Quite mercifully, however, Xigbar is unaffected by this ability, as the accuracy of his attacks is... a bit lacking in this game.
Some Black Magic spells in the first Spellforce can literally be cast from Hit Point (and a little mana). Their effect is the same as another Black Magic spell, which does a huge amount of damage, but cost a lot of mana and have a long cooldown.
In the original Sa Ga Frontier, there's a couple spells that certain characters can get that cost Life Points to buy. Seeing as most characters only get seven or eight of these chances—and once you're out, that character is completely dead until you get to an inn that resurrects (and not all do)—you really have to weigh the pros and cons to get the spells.
Early on in Bloodrayne 2, Rayne comes across a pair of supposedly legendary anti-vampire handguns called the Carpathian Dragons. Since they process blood for ammo, you reload them by draining mooks (which you also have to do to heal yourself, so you now have to balance keeping your health and ammo topped up). The Dragons can still be fired if their reservoirs run empty, but they'll drain blood directly from Rayne - which means that each shot saps a little of your health.
Playing the Vampire race/class in Desktop Dungeons causes spells to be cast from your health instead of mana.
Amea applies this by aversion of Required Secondary Powers—for instance, fire spells burn your hands. You can reduce the cost of spells by finding gloves.
In Fate/stay night, Servants are made of mana. Using their special techniques/Noble Phantasms uses up mana. If they use them too much, or use them when weakened or otherwise injured then there is a risk of burning themselves out. Luckily, most Masters constantly supply their Servants with mana, and only a truly incompetant Master (*cough*Shirou*cough*) would have to rely on Deus Sex Machina to start the mana-transfer.
In the BYOND game NE Stalgia, which is an online game inspired by early 8-bit RPGS, has a warlock class which uses HP for spells. One spell, shift, consumes health from the warlock and restores the health of another party member.
In the Quest for Glory series, you have Health, Stamina and Magic points. If magic is drained, you can't cast spells, no matter what. However, this trope applies to Stamina, which allows you to do anything more strenuous than day-to-day activities (like fighting, running and so on): if Stamina runs out, you get the message that "You are so exhausted that everything you do hurts." Any subsequent use of Stamina will drain your health, and you can, in effect, exercise to death. If this happens in battle, however, you die (due to lacking sufficient energy to defend yourself).
In the Saga of Ryzom, You can customize your attack skills and spells to do this. It's considered an economical choice.
The Dragon's Heart attack in Adventure Quest uses up some of your HP but heals your mana.
Skarlet from Mortal Kombat 9 can throw an unblockable Blood Ball at the opponent, but each one thrown eats up some of her health. Justified because she is literally made of blood.
In Flink, Flink's magic and health meters are one and the same.
The Berserker class in Dungeon Fighter Online literally uses his own blood as a weapon, to the point of dual wielding swords made from his own blood. All of his blood skills (understandably) use hit points instead of mana, and he gets buffs that allow him to drain health from enemies.
In Tales of Maj'Eyal, Reavers and Corrupters can use the skill "Bloodcasting" to cast using HP if they can't afford the Vim cost of their skills, and "Life Tap" to boost their damage for a time at the cost of HP. The only thing that prevents all of their spells from being cast from HP all the time is that they steal the Life Energy of other things, and so can only regain Vim by attacking creatures.
In Star Control II, a ship's crew functions as its hitpoints. The Orz can send crew members into space to board the enemy ship, while the Druge can sacrifice crew members in order to fire their giant cannon more.
Amagon: The most powerful attack in the game is a laser that Megagon can shoot from his body, at the cost of two hit points.
In Athena, athena loses one hit point each time she swings the powerful Flame Sword. This can't kill her, because the sword powers down when she gets low on health.
In EYE Divine Cybermancy, you can install the "Power Converter" implant, which bolts onto the heart. When activated, it converts blood into energy for your other implants or psychic abilities.
In Xenoblade Chronicles, a few character's arts fall into this cathegory. Both Shulk and Dunban have arts that fill a percentage their talent gauge (think on a downplayedLimit Break) at the cost of half of their current HP; and Melia has Healing Gift, which sacrifices some of her HP to heal another party member (and considering she draws the least aggro out of all characters depending on how you play her, this isn't too bad).
Tatsunoko Vs Capcom has Baroque mode, which to execute you must be willing to give up red health (meaning health that can be healed on the sidelines). However, the more punishment you take means the more health you can exchange to power up, and that means the stronger you'll be under its influence.
In a similar vein, in Ultimate Marvel Vs Capcom 3, Nova's Gravemetric Pulse and Gravemetric Blaster attacks become much more powerful by sacrificing red health.
Not exactly, keeping Black 'Mana' in your body will cause this to happen, but it seems that expending all your black magic won't hurt you, so much as it'll hurt the life force of everything else around you. (Casting from Earth's Hit Points?) It also casts from SAN points. So you can end up "Dead. And crazy."
Kid Radd demonstrates the drawbacks of such techniques. When Radd and the others visit a fighting game, Sheena, being an NPC sprite, is unable to inflict or receive any damage from her opponent. Her opponent grows increasingly desperate to damage her, and finally uses an attack that sacrifices some of her life to attack Sheena. The attack fails and because Sheena's opponent has less health than Sheena when the time runs out (i.e., less than full), Sheena wins.
Shomei: Effectively, the Green absorbs the backlash.
In Gargoyles, the Magus taps the magic of Avalon in the episode of the same name, which severely weakens him. While initially it only exhausts him, he ends up casting so many spells this way that he dies as a result.
In Huntik Secrets And Seekers, There is a spell called "Soul Burn" which trades life force for enhanced powers, for a short time. Used in episode 26 by Sophie Casterwill.
In W.I.T.C.H., those who use their magic without being connected to the Heart of Candracar ends up using up their own life energy doing so. Halinor is shown to be exhausted after using some of her magic to protect the Citadel and it's implied that Nerissa's withered state is because of her constantly using Quintessence.
Elita One, in The Transformers episode The Search For Alpha Trion, had the power to stop time at a localized level, but doing so drained her Hit Points.
The Elder Scrolls V Skyrim has a spell called "Equilibrium" which drains your health and refills your magicka. Basically the opposite of a healing spell.