Spilling of blood is a potent force in the working of magic. It may be a token sacrifice, but it may also be the loss of life that fuels the spell. Expect mages who practice Blood Magic to be portrayed as evil, or at least dark grey.
Some blood may be indicated to be more powerful than others. Common types are human blood, the blood of royalty, the blood of a special line, a child's blood, or virgin's blood. Sometimes only a single person's blood has power, and any other blood is powerless.
The other side of the coin is menstrual blood, which is used much less frequently but is associated with life magic when it is used. Except, of course, when it's the Menstrual Menace.
This trope is Older Than Feudalism, with blood and sacrifice being powerful magic in some of the oldest tales. A very old Greek curse was for a witch to run three times around the house she wished to curse while menstruating. Especially potent if it was her first period. In more recent works it is frequently the alternative to Necromancy as the "evil magic".
Often overlaps with Black Magic, The Dark Arts and Cast from Hit Points. See also Our Vampires Are Different and Body to Jewel.
Frequently a part of any ritualistic Blood Bath. When it's the blood itself doing the damage, you have Bloody Murder. When the magic is used to extend the user's lifespan, it's Life Drinker.
May even overlap with Tome of Eldritch Lore, which is often written in man's blood on parchment made of human flesh. Is often a cause for a Superhuman Transfusion.
Examples
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Anime & Manga
In Princess Tutu, Fakir revives Mytho's Cool Sword by cutting his hand, and pouring the blood from the wound into a fountain, and reciting a spell in German while dipping the blade into the bloody water. The various applications of Raven's Blood (like turning Mytho evil) might apply here, as well.
By drinking someone's blood and then standing on a symbol also made of blood (which doesn't have to be theirs), Hidan can turn himself into a living voodoo-doll for his opponent.
In Fullmetal Alchemist, Ed and Al used their own blood as an ingredient in their attempt to resurrect their mother. When that failed miserably, costing Ed a leg and Al his entire body, Ed used his blood to draw a seal on a nearby suit of armor, binding his lost brother's wayward soul to it. This is justified by noting that the iron in the hemoglobin bonded with the iron of the armor, and the destruction of the bloodseal would kill Al.
Later, when Martel is killed while inside Al, some of her blood runs down and over Al's blood seal, making him black out. However, this also allows him to remember what happened to him beyond the Gate.
The transmutation circle to generate a Philosopher's Stone needs to have five "crests of blood" made from killing. For the nationwide transmutation circle, five gigantic military engagements which could be more aptly called butcherings created the five crests needed to cover all of Amestris.
In the 2003 anime adaptation, Mustang ends up creating a transmutation circle out of his own blood in order to defeat Pride.
In The Movie based off of the 2003 anime, the circle to open the Gate only activates when it gets in touch with blood. Edward has a little blood on his glove the first time he opens it. The second time Hohenheim gets mauled by Envy, while in Amestris Wrath dies.
Some of the magic in Mahou Sensei Negima! works like this. At one point, Negi bites his thumb hard enough for blood to pour out, then creates magic circles for The Thousand Bolts (the ultimate lighting incantation) and pulverizes a mountain sized rock. Blood of powerful mages also seems to carry a certain amount of energy in it if Evangeline is to be believed. If.
Deadman Wonderland has an interesting spin on Blood Magic: the eponymous Deadman are capable of using Branches of Sin, a magic that allows them to weaponize their blood, such as Ganta's bloody hand blasts, Seiji's arm blades, and Karako's blood armor.
A vampire planeswalker known as Sorin Markov is said to use sangromancy (blood magic), which enables him to drain the lifeforce of other beings, place curses on enemies, and possess the minds of others. It is also implied that he can leech mana from opponents. How exactly he uses blood is not stated.
Sorin's grandfather Edgar Markov turned himself and Sorin into the first vampires on the plane of Innistrad using a combination of sangromancy and a Deal with the Devil.
Long before he showed up, the ogres of Kamigawa, a Japanese-themed world, used blood oaths to bind oni (demonic spirits in this setting) to their service. This was represented mechanically by ogres with abilities that "turned on" when you also controlled a demon, and demons with drawbacks that "turned off" if you controlled an ogre.
Comics
In The Sandman: A Game of You, menstrual blood is used to power a spell to send the characters to the land of the dead and come back alive ( mostly). The spell is repeated with blood from a cut palm in Death: Time of Your Life.
Nico from Runaways can only summon her Staff of One when she is bleeding. Menstrual blood also works in this case.
Another parody happens in Lenore the Cute Little Dead Girl, when she pricks her finger and spills a drop of blood on a doll. It turns out to be a vampire that was cursed to be an inanimate plaything, and her blood broke the curse. Unfortunately, he realizes he's still in a doll's body because the curse didn't break properly; she'd been embalmed.
This is how Atrocitus created the Central Power Battery for his Red Lantern Corps. In addition, he is capable of scrying through arcane blood rites.
The Teen Titans villain Brother Blood leads a cult who focuses magic through bloodletting (yes, he's a little different from the cartoon version). One of his best-known acts was resurrecting Raven to be his bride (it was basically the writers' way of bringing her back to coincide with the success of the cartoon).
Not a major example, but in one Batman Detective storyline, Zatanna had been shot in the throat and nearly drowned in a death trap courtesy of the Joker (don't ask why he didn't just shoot her in the head). She avoided bleeding to death by using her blood to write out a healing incantation, made extra-strong by its medium.
Conan the Barbarian once battled a witch who used a particularly nasty curse triggered by blood. Only a single drop of spilled blood was necessary to cause her victims to age rapidly and die within seconds, leaving behind a withered corpse. The only protection against the curse were magic talismans owned by three brothers who were the witch's enemies.
Red Hood and the Outlaws: Red Hood's All-Swords get extra power from his blood. Or, something like that. It hasn't been explained yet.
Fan Fiction
Shinji in Points Of Familiarity bites off his finger and sacrifices it to ... something to create [Masks].
Subverted in-universe in The Dresden Files fanfic Fair Vote. The protagonist uses tea as a metaphysically symbolic substitute — and it works.
A Growing Affection has both the powers of the Blood Drinker demons, and Naruto's Blood Clone Jutsu.
Escape from the Hokage's Hat has Naruto with multiple seals placed on him (by various Konoha ninja) to limit his growth. They were all made with blood. His own. After many a beating. Judging from a bit of dialogue between Tsunade and Shizune when they find out, placing a blood seal on someone is implied to be excruciatingly painful. Naruto got them between the ages of 6 and 9.
Films — Animation
In The Princess and the Frog, Dr Facilier turns Naveen into a frog as to have an easy means of extracting blood for a voodoo talisman that turns Naveen's Beleaguered Assistant Lawrence into a copy of the prince for his own ends.
Films — Live-Action
In the first Pirates of the Caribbean, Will's blood was needed to break the curse on Barbossa's crew. A blood sacrifice was required from every cursed pirate, but they had cast Will's father overboard before learning this, so Will's blood, as the closest living relative, was used as a suitable substitute. Once Jack cursed himself, he also added his blood to lift the curse "at the opportune moment".
In Silent Hill Christabella unintentionally invokes this trope when she stabs Rose in the chest. The blood gushing from the wound destroys the church, allowing Alessa to enter. Then all hell breaks loose.
In Blade, it's of little surprise that the ancient vampire artifact runs off of blood. Blade is strapped into a huge bloodletting device that causes his blood to fill up all the magical symbols below him, rather similar to the picture above.
The page image comes from the first Hellboy film; Ilsa and Kroenen kill their guide in order to resurrect Rasputin.
Literature
Melisandre of A Song of Ice and Fire uses blood magic to cause the death of Renly Baratheon, and claims credit for the deaths of Robb Stark, Joffrey Baratheon, and Balon Greyjoy, but whether that's truth or fiction is very unclear at this stage. The maegi Mirri Maz Duur also uses blood magic to "heal" Drogo and the favor is returned by Deanerys when she burns the maegi alive as part of the magic to awaken her dragons.
Mercedes Lackey loves this concept, and blood mages are frequently villains in her books. In the Heralds Of Valdemar books, at least, it's the sort powered by death, not the red stuff itself, and gathering it from anyone else is exclusively villainous. Good-aligned Mages and Priests occasionally use their own death energies to really make a Heroic Sacrifice stick, though.
In Stardust, the witches use the hearts of living stars to prolong their youth as a form of blood magic.
In Harry Potter And The Goblet Of Fire, Voldemort uses Harry's blood to reconstitute himself. This later turns out to have been a horrible idea since Blood Magic mixed with the Power of Love to give Harry a Deus ex Machina resurrection in book seven, and ultimately ends up being Voldemort's undoing.
And in The Half-Blood Prince, one of the enchantments used to protect a Horcrux can only be lifted with blood, if only because the caster wanted to make sure no one could enter without hurting themselves. Dumbledore is actually disappointed that Voldemort couldn't come up with something more inventive.
Unicorn blood is so potent that drinking it can preserve the life of a somebody who is already dying, but the act of taking it is said to curse the killer with a "half-life".
King Of The Water Roads - Magic "usually" takes a toll in blood and pain to cast, according to the only trained magic-user seen in the book.
Terry Pratchett's Discworld parodies this with the Rite of Ashk Ente, which summons Death. It's implied that this is supposed to require a human sacrifice and dozens of magical foci, but magical refinements mean that it's now possible with only an octogram, three small bits of wood, and 4 cubic centimetres of mouse blood.
This is further refined to just needing the octogram, two small bits of wood and a fresh egg.
A straighter example is in Carpe Jugulum, where Granny Weatherwax uses this kind of blood magic against vampires by "infecting" them when they feed on her.
Granny: I ain't been vampired, you been Weatherwaxed!
In the Dragonlord series by Joanne Bertin, blood magic is frequently employed by the antagonists. In The Last Dragonlord, the Big Bad uses the lives of prostitutes to charge a magical artifact. He then sacrifices a member of a specific bloodline in order to attempt to enslave one of the titular dragonlords. In The Dragon and the Phoenix, the blood of the high priest is used to bind a dragon, and the magic of the dragon is used in turn to imprison and harness the power of the phoenix.
In Graham McNeill's Warhammer 40000Ultramarines novel Warriors of Ultramar, the Ultramarines contact another Chapter, the Mortifactors, and find their use of Blood Magic rather abhorrent.
In Lee Lightner's Warhammer 40000: Space Wolf novel Sons of Fenris, Cadmus uses Blood Magic, killing one of his officers who's guessed too much, to open a portal.
In James Swallow's Warhammer 40000Blood Angels novel Deus Encarmine, a knife tastes Rafel's blood and sends it back, and a Word-Bearer, by tasting it, can judge his age.
Tamora Pierce makes use of this both in Circle of Magic and the Tortall Universe. However, it is made abundantly clear that the only approved use of blood magic is when the mage uses his or her own blood.
In the Young Wizards series, healing spells require blood (that isn't from the patient), usually the healer's own blood.
This is one type of magic used in the Evie Scelan novels.
In Holly Lisle's The Secret Texts trilogy, each of the three schools of magic use blood/flesh sacrifices. The origin of these sacrifices reflects where the magic lies on the good to evil scale. Falcons use their own blood and are good. Wolves use others present at the time as a sacrifice and are bad. Dragons are able to use whole populations at a distance as their sacrifice and are Super Scary Evil.
In Mistborn, Hemalurgy is a magical art that involves killing a victim and transferring certain of their qualities to a recipient (the precise process involves both getting stuck with a metal spike, often a large one). Who the victim is normally doesn't matter, but if you want to transfer magical powers, you do need to find someone who has them to begin with. The precise composition of the spike and where it is stuck in the recipient's body determine precisely what qualities are transferred.
In Dan Abnett's Gaunt's Ghosts novel Blood Pact, the witch makes extensive use of this. On both the prisoner and Maggs — she didn't realize she had gotten blood from both of them at first, but when she did, she tried to use Maggs to assassinate the prisoner.
The Canim ritualists of Codex Alera drain the blood from sentient beings (living or freshly dead) to fuel their sorcery. They demonstrate a wide range of abilities including summoning storms, shooting lightning bolts, conjuring flying demon-things to keep airborne enemies out of the upper atmosphere, unleashing poisonous gasses, and other similarly creepy and destructive things. The most decent ritualists, such as Marok, use only their own blood to fuel their magic; the ones who don't tend to lean towards being Evil Sorcerers.
This is a severe limitation on their power, since the quantity of blood is very important. The storm that covered the Canim's invasion is mentioned as having cost millions of their own lives to pull off. For a long time after the Canim invaded no magic was used, because they had an insufficient source of it. When they started using it again it was revealed to be mostly fueled by Aleran slaves who had defected to the Canim and given permission to be drained after other Alerans killed them.
In Orson Scott Card's Hart's Hope, blood is the essential source of magic: the more precious the blood, the most potent the magic. Kill a rabbit, and you can stop a stew from boiling over. Kill a deer and be able to turn invisible. Then kill a man to get real power. In order to exact her vengeance, Queen Beauty therefore kills her own newborn daughter, which allows her to acquire powers that make her able to subdue the gods.
The bloodline of Celtic god Cernunnos gives his descendants magical abilities they can use against him in Urban Shaman by C.E. Murphy.
In The Dresden Files, the main character sometimes uses traces of people's blood to locate them, but it only works if it's very fresh. In Small Favor, the Denarians use the blood of an unknown person to fuel their Hellfire-powered pentagram barrier.
Kate Daniels draws blood wards, magic circles drawn in the caster's blood which draw their strength from the blood itself. They can also be broken by someone of similar blood, a loophole Kate exploits to her own advantage. It is also found out that she can create armor out of her blood as well as weapons, but they disintegrate within seconds. A power she received from absorbing The Scarlet Star.
In the Night Watch universe, the lower levels of the Twilight will quickly drain your energy; if you need to get out fast, spilling blood will do the trick.
In Andy Hoare's White Scars novel Hunt for Voldorius, the Alpha Legion uses it. The White Scars find bodies drained of blood.
In "The Phoenix On The Sword", Thoth-Amon, immediately on regaining his Ring of Power, uses the blood of the man he murdered for it to summon a powerful abomination against his old tormentor Ascalante.
Blood is noted as being particularly potent for drawing spell forms in the Sword of Truth series, though it's trickier for lasting spell forms as blood will eventually dry out and thus weaken the spell. The People's Palace in D'Hara is a spellformnote which, for reference, saps the magic of any non-Rahl wizard and bolsters the magic of any wizard of the Rahl bloodline that manages to get around this through the novel method of leaving the blood inside people. The blood therefore is always fresh, and people are always allowed to travel through the halls of the palace, keeping the spell very powerful.
In L. Jagi Lamplighter's Prospero Regained, blood can give power and life to those in Hell. Kinda awkward when hellish mosquitoes have figured out you are alive and so have blood.
In Teresa Frohock's Miserere: An Autumn Tale names written in blood are particularly potent magic.
Subverted in Count and Countess, where a large part of the story hinges on the main characters' belief in the magical properties of blood. Turns out there are no magical properties.
In The Orphan's Tales, starlight is really a blood of the sky and can be used to perform some kinds of magic, like Shapeshifting.
In Magic in the Blood, a villain steals some of Allie's, which seriously worries her.
In Stephen King's On Writing, he describes some symbolic associations of blood with life and death, sin and redemption (through sacrifice), and how they're used in Carrie as a parallel to the titular psychic's emerging powers and rampage.
Malkar and Vey Coruscant in The Doctrine of Labyrinths are accomplished practitioners. They have been using this to keep themselves young and vital for who knows how many years. One of Vey's many epithets is "Queen Blood."
In Tales of Kolmar demon-summoning mages and those aligned with them always have to give the demons some of their own blood. On the heroic side, a mage discovers that when she touches her own blood to a dragon's soulgem her demon-fighting powers are greatly enhanced, to the point of being able to dispel minor ones with a touch.
Live-Action TV
In Doctor Who, in the episode "The Christmas Invasion", the alien Sycorax use "blood control" to hypnotize a third of the population of Earth. Technically it's Magic from Technology, but one of the UNIT characters commented that it appeared like they were casting a spell. The Sycorax reverse-engineered their technology from invaders, and think of it as magic. ('Sycorax' would seem to be taken from The Tempest, where it is given as the name of a dead witch, late mistress of the island and mother to Caliban.)
In Legend of the Seeker, Darken Rahl used Blood Magic at least once so far, using it as magic ink to write a message in a book, which then caused it to appear to a 'paired' book one of his military field commanders possessed (sort of a magic 'instant message' or 'telegram').
In the novels, Darken Rahl uses a kind blood magic in the form of consuming the brains and testicles (Squick!) of a young boy loyal to him to summon a creature of the underworld to ride and thus can travel anywhere very quickly. The travel journals were introduced later in the series and did not require blood to operate.
Also in the books, it's mentioned that blood is particularly potent for things like drawing spell forms, though usually other things like Wizard's Sand also work for spells. The biggest blood-fueled spell form in the series is the People's Palace. The trick here is, the blood is still in the people going through the palace, which is how it's kept potent when just blood would have dried out and lost its effect long ago.
Dawn's blood is used to open a portal. The emphasized plot point that Buffy and Dawn have the same blood is the key to allowing Buffy to save the world.
The trope is specifically lampshaded by Spike in the same episode.
Xander: Why blood?
Spike: Blood is life, lackbrain. Why do you think we eat it? It's what keeps you going, makes you warm, makes you hard, makes you other than dead. 'Course it's her blood.
Angel uses his blood to open a portal, and is later needed to close it again.
Blood of a Slayer, if drunk by a vampire, gives the latter great powers and Nigh Invulnerability for a while. In Season 3, this is used to heal Angel.
Willow uses deer blood in her magic to resurrect Buffy. Specifically, she sits by a creek until a baby deer comes along, before she draws a knife and kills it for its blood.
In season 9, Willow cuts Connor's chest with the scythe to use his blood to tear a hole in the fabric of reality.
In Season 7, blood opens the Hellmouth. Acting on the orders of the First posing as Warren, Andrew kills Jonathan to try this. Except it doesn't work: Jonathan is anemic.
Gachnar is summoned when Oz's blood drips on a painted talisman.
In Angel, a demon from another dimension tries to contact Jasmine with flesh magic. It's even nastier than it sounds.
Angel is temporarily made stronger after he drinks the blood of Hamilton, the Senior Partners' liaison, during the Grand Finale.
Spoofed in "Reprise" where two Wolfram & Hart employees are following a list of instructions for a goat sacrifice.
(reading from a booklet) "Make sure all troths are securely fastened and sacrifices tilted as shown in diagram F-12 to ensure full drainage into sacred offering bowl. Using a clean, diagonal motion, slit throat of sacrifice with the pre-blessed ceremonial dagger provided... I didn't see that in the box."
In Supernatural, Azazel's lackeys used a squicky blood fueled ritual to communicate with him. Brady does the same to communicate with his master, Pestilence.
In the fourth and fifth seasons, blood can be used to construct a sigil that will temporarily dispel an angel from the area.
And in the fourth season finale, it turns out the blood of the first demon, Lilith, must be spilled to complete a ritual to release Lucifer from Hell.
And the Special Children got their powers from Azazel bleeding into their mouths.
A season five episode had Sam, Dean and Gabriel bound to a location via blood.
The sixth season finale had the gate to Purgatory open using a cocktail of virgin blood and blood from a Purgatory resident. They are really fond of this trope.
In Xena: Warrior Princess, the evil god Dahok requires the loss of "blood innocence" to bring forth his evil offspring, so his followers trick Gabrielle into committing murder. In this case, it's the loss of life that fuels Dahok's power, but Gabrielle's hands are also covered in blood.
In Merlin, blood magic shows up in one of the spells of Nimueh in season one.
L Ater, Morgana uses the Cup of Life to create an immortal army by filling it with blood.
In Friday The 13th: The Series, almost every single one of the cursed items must be powered by a death to derive their benefits.
In Star Trek: Deep Space Nine when Kai Winn is attempting to read the Book of the Paugh Wraiths (essentially the Bajoran Necromicon) it appears to her to be a blank book. Another priest catches her at it, is horribly shocked at finding his superior (essentially the Pope of his world) doing such a thing, and starts to run from the room to denounce her to the other priests. She picks up a letter opener and stabs him; his blood falls on the book and—-what do you know, the writing becomes visible.
Myths & Religion
In The Bible, the Passover where the Angel of Death came to kill the firstborn children of Egypt, and the Jews painted their doors with lamb blood so the angel would know not to kill those firstborn children.
The mythical account of Shaka Zulu. In exchange of limitless power, Shaka is said to enter a pact with the witch doctor Issanoussi, who demands the death of Shaka's pregnant fiancee Noliwe — and this turns out to be just the down-payment for "blood medicine"
"The medicine with which I inoculated you is the medicine of blood; so if you do not spill blood in abundance, it will turn against you, and it is you whom it will kill" (Chaka the Zulu — Thomas Mofolo)
The Aztecs believed that the sun needed to be fed on the blood of sacrificial victims daily to prevent it from dying.
The Mayans practiced ritual bloodletting (and sacrifice, too, but only when they were desperate). They also had a ritual ball-game that was not only ruthless, but ended in the winning team being sacrificed at the conclusion.
In some parts of the West Indies, it used to be common to use animal blood at a boats launching ceremony. Squeamish Americans and Europeans just use champagne, the wusses.
A common story from the German and Dutch North Sea coast is, that in the past people made live sacrifices when building dykes against floods. Burying a dog alive under the new dyke would work, but a human child was considered preferable. The important part was, that a living thing had to be given to the spirits.
Tabletop Games
In Vampire The Masquerade, all vampires gain their abilities, obviously, from consumed blood. However, a few clans take this one step further by developing a blood-based system of sorcery stronger than the hedge magic any human can learn but weaker than the Sphere Magick of Mages. The most well-known practitioners of blood magic are Clan Tremere, whose original members were Mages in life.
Vampire The Requiem carries this on with various forms of Blood Sorcery, the two most predominant being Cruac (a humanity-stifling, ritualistic art practiced by the Circle of the Crone) and Theban Sorcery (a series of "miracles" that require appropriate sacrifice, practiced by the Lancea Sanctum). A third example is the bloodline power of the Gethsemani bloodline, which allows them to make mortals spontaneously suffer stigmata-like wounds; the blood from these wounds has magical effects on a vampire who drinks it.
Hunter The Vigil features the Cainite Heresy, remnants of an ancient cult that weren't too happy after being dicked around with by a vampire claiming a direct connection to God. Somehow they got their hands on vampiric blood magic, and created the Rites of Denial, special powers meant to deny vampires their innate advantages.
Changeling The Lost has Tokens, magic items that activate with a simple burst of Glamour or with a simple Wyrd check. Either of these can be foregone- and the items can be used by mortals- by paying the "Catch," a "dread cost." Examples? One, a minor thing that empowers your car, requires you to run your car on a pint of your own blood (one point of lethal damage); another one, a more potent one called a Pledge Stone, requires you to rip out your own tongue and burn it (one point of lethal damage and you don't have a tongue).
And in Mage The Awakening, blood sacrifice (namely, draining a being, including a human, of its blood until it is dead) can be used to replenish Mana. The use of blood can also correspond to the powers of Mastigos or Thyrsus mages.
Geist: The Sin-Eaters has the Stygian Key, a source of power based around manipulating the raw essence of death. Few Sin-Eaters obtain it, as you need to drink from several rivers of the Underworld and make a deal with a Kerberos to learn its tricks. But even then, you can only use any of the Manifestations associated with the Key by performing a sacrifice first. There's also the Stigmata Key, which is much more common and allows the Sin-Eater to control blood and ghosts. Some of its Manifestations require a portion of blood in order to function, and it can sometimes be empowered by shedding blood when you don't need to.
Shadowrun supplement Aztlan. Aztlan mages can use Blood Magic: spilling a human's blood to enhance spellcasting and summon Blood Spirits. Extremely evil, restricted to Non Player Characters. How restricted? If any player character starts learning Blood Magic from any source whatsoever, of their own free will, their character sheet is now that of an NPC. It's just that dark.
Expanded in the supplement "Street Magic". Adepts can benefit from blood magic, with at least one of the new metamagics revolving around cannibalism.
Earthdawn. Blood charms are used to seal Blood Oaths and gain magical benefits.
In earlier editions, a magic user used his own blood as an ingredient in creating a homonculous, possibly inspired by the Sinbad example.
One of the material components for the Cacodemon spell (which summoned a powerful demon) was a bowl of mammal blood, preferably human.
The Blood Mage prestige class allows a spellcaster use their own blood to improve their spells. There's a Wizard paragon path along much the same lines in Fourth Edition.
In the book "Complete Arcane" for 3.5, the Prestige Class of "Blood Magus" is described as "formerly deceased spellcasters who, when returned to life, gain an understanding of their blood's importance..." They can create a homonculous as explained above, but they have many other abilities as well, a particularly frightening one being "Bloodwalk". With Bloodwalk, a Blood Magus can teleport by entering the body of a living creature the same size or bigger, either leaving peacefully or exploding out violently.
The Maho Tsukai from the Oriental Adventures can, like the 3rd Edition Blood Magus, use his own blood as a replacement for the material components for spells. He can also use someone else's blood, although that takes more time and more blood. Finally, he can use large amounts of blood (enough that it's represented by Constitution damage rather than regular hit point damage) to apply metamagic to his spells without using a higher-level spell slot.
Many Forgotten Realms spells are blood-related, like Bladethirst, Bloodstars, Blood Lightning, Beltyn's Burning Blood and elven Blood Dragon, and there are less specific mentions of elven blood magic in novels. Lots of other spells introduced in FR, while not inherently blood-themed, need as a spell component any blood (Revenance, Nulathoe's Ninemen), blood of specific creatures (e.g. red dragon for Daltim's Proof Against Fire, dwarf for Semipermanency) or the caster's blood (Bone Javelin, Elminster's Evasion, Fellblade, Immunity to Undeath, Lich's Touch, Mummy Touch, Phezult's Sleep of Ages, wizardly version of Negative Plane Protection). Components of magical inks, elixirs, etc appearing in Realmslore include blood of various creatures more often than not.
Ravenloft was created when Strahd von Zarovich murdered his younger brother Sergei, in a blood sacrifice intended to magically restore his youth.
Dark Elves in Warhammer use blood magic. Their sorceresses can sacrifice wounds to get more magic dice, and they have large blood cauldros that imbues nearby units with the blessings of their god Khaine (also known as the bloody handed god).
In Legend of the Five Rings, practitioners of maho fuel their spells by spilling blood, often their own. The explanation given is that kansen, malevolent spirits, are attracted to the caster by this act and will then exert their influence over the world in accordance with the mahotsukai's wishes.
In Deadlands: Reloaded the Whateley Blood edge allows you to spill your own blood in order to gain "power points".
GURPS: Thaumatology codifies this: you have to spill enough blood to drain 20% of your Hit Points in order to get + 1 to casting. Unfortunately taking damage also makes it harder to cast a spell, so without the High Pain Threshold advantage there's not much point to it.
In Exalted, all Necromancy spells require a sacrifice of blood as part of the casting. In most cases, this is nothing more than a token sacrifice, and The Black Treatise, the Necromancy sourcebook, notes that necromancers tend to keep bandages handy. For the most powerful rituals, though, more gruesome sacrifices are required; Necromancy is very much Black Magic in the world of Exalted.
Scion features the Aztec Pantheon as one of its sample pantheons. As a result, Aztec Scions get access to the Itzli Purview, which allows the Scion to gain power through blood sacrifice. These powers range in potency from "sacrifice some of your blood to gain Legend" to "rip out someone's heart and plug it into your own chest, where it catches fire and grants you new-found vigor."
Likewise, Scions of the Aesir get access to Jotunblut, which allows them to use the blood of the giants to make mortal companions stronger. At the highest levels, it allows mortals to turn into giants themselves.
A character with Advance Shape Shifting can use their own blood to create creatures which have some of their powers. These creatures are NPCs and not under the creator's control.
A character with Shape Shifting and Conjuration can create items out of their own blood. A character with Advance Shape Shifting and Conjuration can create items out of other people's blood.
Taken to the extreme in Final Fantasy Tactics, in which human sacrifice is used on a titanic scale by Lucavi, inciting a war with hundreds of thousands of casualties to spill enough blood to raise the Big Bad Altima.
Almost literally in Final Fantasy Tactics A 2, where the skill "Blood Price" lets you use HP to pay for magick instead of MP. The catch? The HP cost is double the normal MP cost, and you can't use your MP at all with this skill equipped. However, the simple convenience provided by it more than outweighs the drawbacks. Not to mention that you still get HP back if you use it for healing spells.
In the Dominions series, spells from most schools of magic are cast using "gems" which are basically elemental forces (fire, water, etc...) concentrated into portable form, and the casting prices of spells are listed in these gems. Instead of gems, however, one school of magic lists spell prices in blood. These prices are listed in increments of one blood slave, each of which must be drained completely empty, with high-level spell costs running into the hundreds. Blood magic is conspicuous in battle, where mages or priests are surrounded by a white-robed flock they systematically stab, especially if the mages start losing and set the slaves to charge into the spears of the enemies while they themselves flee. Dominions is not a happy series.
In God Of War II, at the Temple of the Fates, Kratos is required to capture a translator to read the incantation needed to open his path. Said incantation ends with the reader offering his blood as a sacrifice. Cue Oh Crap just before Kratos bashes the guy's head in against the altar, his blood draining into a pattern etched onto the floor.
Sort of used in Devil May Cry. When you kill monsters you collect their blood (which conveniently crystallized into red orbs in contact with air) to upgrade your magic powers. Justified in that there is a bounty system in placed by some nebulous god of good.
Also, in Devil May Cry 3, Arkham used the blood of Dante, Vergil, and Lady to open up the portal to the demon world.
Clive Barkers Jericho has Wilhelmina "Billie" Church, a powerful blood mage. Her blood magic and abilities play a very important part in the game's story, and her spells can bind enemies and set them on fire.
The Sumerian Demons Inanna and Ninlil also rely on blood magic (referred to as sanguimancy ingame). While Inanna uses the blood of Innocent people bound to torture wheels, Ninlil has herself locked within an Iron Maiden to spill as much of her own blood as possible.
In World of Warcraft, according to the background warlock's life tap ability (convert health to mana) works exactly like this (they sacrifice their own blood to gain mana), using it to summon most of their demonic pets, empowering their weapons or, curiously enough, a number of beneficial effects like summoning party members or Healthstones which can be consumed like a potion.
Warcraft III introduced Blood Mages, but despite the name they don't practice any of this; most of their spells are fire-based. Lampshaded by one you meet in the Blasted Lands, who says Blood Mages "eat normal food like normal people."
In Guild Wars a Necromancer's blood magic skills often have you sacrificing health to achieve an effect. The same class also uses Death magic, which exploits your dead opponent's corpses (and therefore blood) for similar effect, and their innate Soul Reaping ability, which heals you upon an enemy's death.
Blood magic also has you drain health.
This is one of the ways a Servant is summoned in Fate/stay night. You take some chickens, put them to sleep, kill them, draw out their blood and make a magic circle. Then, you can either use a catalyst for a specific hero or just the general term of inducing madness in order to get a Berserker. Rider and Caster in Fate Zero were also summoned in this manner, and it's more noted upon there as well.
In The Elder Scrolls V SkyrimThe only way to open the old Blades headquarters in Skyrim is with the blood of a dragonborn.
Blood Mages. Their magic allows them to Cast from Hit Points, drain the life energy of others to fuel their magic, control minds, boil others' blood in their veins, and sometimes even command demons. Despite this, they're not always evil, but most are, and even the good ones tend to be treated as if they are, since the most common method of learning it is through a pact with a demon.
What's important about Blood Magic is that it is pure life force, and can thus provide plenty of energy. And it isn't limited to Mages, either. Anybody can learn how to tap into it, even a warrior (exemplified by the Reaver, who uses a different path, but one which still uses the power inherent in blood as energy). Since it was originally taught by an Old God (demons are the only ones who still remember it in most cases, however), it may not even be evil. The Soldier's Peak DLC introduced Avernus, who had worked out how to change blood as the fuel source for the Taint. And then there are the Chantry Templars, who gain their abilities from the energies within lyrium (apparently - that it is possible to wield what is essentially Templar-created magic without using lyrium might speak to some other source), all without being Mages. While the Fade is probably the easiest source to learn how to use (and the one with the least reliance on an outside source to power one's spells), it's definitely not the only source. Indeed, it might be that anybody could learn how to use magic, as long as they were willing to use something else as fuel.
The state religion of the Chantry demands that all persons with magical abilities have to permanently join the Circle of Magi and be confined to their towers where they are constantly monitored by templars who are to slay them at the first sign that demons have possessed their bodies. Every mage who tries to escape from the Circle of Magi or refuses to join is treated as being potentially possessed and killed. Since blood magic is not officially sanctioned by the Chantry, blood mages share the same fate as all other apostates.
The Joining - Grey Wardens gain their powers by drinking darkspawn blood. The cost to this is the Darkspawn Taint eventually will overtake every Grey Warden, turning them into a Darkspawn themselves. They avoid this happening by suicidally venturing into the Deep Roads and trying to kill as many Darkspawn there as they can when they feel the taint has gotten too strong.
Another BioWare example is in Jade Empire. The defenses of Dierge were broken when the sacred fountains were polluted with human blood. Useful in the nastiest way possible if you take the Closed Fist option.
The Dark Knight class that appears in various Final Fantasy games has the ability to sacrifice health to either empower or fuel their attacks.
Taken to the extreme again by Vladimir the Crimson Reaper, a "hemomancer" in League of Legends. Not only are all his abilities blood-related (including draining the blood out of his opponents and into him and briefly turning into a near-invulnerable pool of blood) but he fights with an orb of blood which hovers in his hands that he shoots at his opponents.
Grimoire Weiss of Nie R absorbs the blood of fallen enemies to create magical constructs for his partner to attack with.
Shao Kahn created Skarlet out of blood collected from various battlefields in Mortal Kombat 9.
In Supernormal Step, Jules and Van practice blood magic. However, Jules made Van swear not to use blood magic without his guidance. This almost certainly will affect the plot later.
Biscuit begins with a character opening an envelope sealed by blood magic.
In Drowtales, blood sorcery is a branch of mana manipulation, and the protagonist Ariel is especially vulnerable to it. Syphile, her abusive caretaker, has an affinity for blood magic and uses it against Ariel at few times.
Later on we get a dark take on the idea when a human king begins a campaign of kidnapping and brutally killing elves so he can bathe in their blood, hoping to gain immortality.
In Sluggy Freelance, the blood of the innocent awakes a powerful magic spirit in Torg's sword.
The Erlkönig in Roommates used his own blood to seal his son's magic and memories (symbolically his left eye). What power the guy has that a the blood of a blood related faenoble was needed is anyone's guess (maybe the Erlkönig is just a show off).
Web Original
Fey has used this in the Whateley Universe against a dark mage who was trying to enslave her.
Western Animation
In American Dragon Jake Long a drop of Jake's blood is used to restore the Dark Dragon's full strength.
