Blood is a powerful symbolism, and sometimes even has mystical powers. The Blood Oath makes use of this to make a commitment that can't be broken. (Or if it is, it's crossing the Moral Event Horizon.)
Bob has a secret he needs to get off his chest, but doesn't want it to be made public. Alternatively, he needs to be certain Al will do something for him. Either way, this is so serious that it requires making a blood oath. After all, once you're bound by blood, you can't break that promise, even if you're a villain.
In one common version, Al and Bob become blood brothers, cutting their hands open and shaking on the deal. (Don't Try This at Home; these rituals carry a risk of transmitting bloodborne infections like hepatitis and HIV.) A lighter version is the Spit Shake.
In another, Al cuts his hand open, and simply lets the blood drip to the ground. This is sometimes highlighted by the phrasing of the oath: "If I break my word may the Earth drink my blood!" Then giving the Earth a 'taste' of your blood so that it knows who to look for if you fail to keep your oath. Sometimes this version is used to swear an oath of vengeance- and making it clear that you mean business.
This is a subtrope of The Power of Blood. If it uses blood and binds you to an oath, then it belongs here.
Can overlap with Couldn't Find a Pen, which is any kind of writing in blood. Contrast with Heroic Vow. See also Geas and Rash Promise. May or may not involve actual Blood Magic. When someone has broken this vow so thoroughly they become ostracised they're The Oath-Breaker.
- Chrono Crusade: It is explicitly said that when Chrono made a contract with Mary Magdalene, his first contractor, that she had to give him some of her blood to complete it. It's implied (using vampire imagery) that this was also the case with his contract with Rosette.
- Mobile Suit Zeta Gundam: Paptimus Scirroco seals his loyalty to the Admiral Jamitov Hymen and Titans with a bloody thumbprint. This does not stop him from murdering Jamitov and taking over the organization.
- A variant. Naruto swears to Neji on Hinata's blood during the Chunin exam preliminary battles for the final test, "You... are going down", promising to defeat him. He makes the same gesture at the beginning of their fight in the finals.
- Summoning contracts are also typically sealed in blood, with a small blood offering made each time the summon is invoked.
- This is also all one really has to do to exchange mana in the Nasuverse as well. Again, sex is more fanservice.
- It's implied that this is the traditional way in Negima! Magister Negi Magi to induce a Pactio. But that's messy and doesn't have nearly enough fanservice, so they kiss instead.
- In Monstress, it's shown that Arcanic wedding ceremonies involve having both participants cut themselves and bleed into the same cup, followed by them both drinking the mixed blood, representing how they've symbolically become one.
- Superman storyline The Hunt for Reactron provides one example. When they were young, Thara saved Supergirl's life from a giant bird of prey. Since Thara had cut her finger with the crystal-like petal of an alien plant, Kara grasped her hand, mixing her blood with hers, and swore they were bonded as blood sisters from that point on.
Kara: Here— Mix my blood with yours.
Thara: What're you—?
Kara: I don't know if it was voices or Rao or luck, but you saved my life. As did this flower. As long as we each hold onto a piece of it, we'll be sisters.
Kara: Blood sisters. Bonded... Best friends. Forever. No matter what.
- In Alexander Afanasyev's "The Soldier And Death", a soldier imprisons a band of demons in a magic sack and lets them out after they swear to leave the tsar's palace and never return. When the demons are fleeing, though, the soldier grabs one of them, slices his wrist and forces him to write a servitude contract using a pen dipped in the demon's own blood.
But the soldier was no fool, and he grabbed one old devil by the leg. And the devil hung gibbering, trying to get away. The soldier cut the devil's hairy wrist to the bone, so that the blood flowed, took a pen, dipped it in the blood, and gave it to the devil. But he never let go of his leg.
"Write," says he, "that you will be my faithful servant."
The old devil screamed and wriggled, but the soldier gripped him tight. There was nothing to be done. He wrote and signed in his own blood a promise to serve the soldier faithfully wherever and whenever there should be need. Then the soldier let him go, and he went hopping and screaming after the others, and had disappeared in a moment.
- In Coventina's Crown, Morain cuts her palm, lets the blood drip onto fire, and swears by her own blood to bring her kidnapped brother back.
- At the beginning of Double Agent Vader, Anakin has an epiphany about the course his life has taken, and swears a blood oath that he will bring about the destruction of Palpatine, "the downfall of his Empire, and the collapse of his power, and the final defeat of all his plans". With his body sealed up in the Darth Vader life-support suit, it requires a bit of doing to get the blood to make the oath.
- A Growing Affection has the Blood Oath Jutsu, the user swears an oath, sets a condition for breaking said oath (two examples in the story are memory loss and death), and then writes the kanji for Honor over his or her heart in his or her blood. Breaking the oath will cause the condition to occur. Only one person in the world has ever successfully removed a Blood Oath seal, and she has less than a fifty percent success rate in doing so.
- In The Devil and Daniel Mouse, Jan signs her contract with her own blood. It's the one thing in her first encounter with B.L. that makes her hesitate, but only until he basically threatens that it's now or never.
- In We're Back! A Dinosaur's Story, this is how Louie and Cecilia join Professor Screweyes' circus. At first the contract is blank, but once the Professor pricks Louie's finger with his pen and the drop of blood touches the paper, it magically fills out, with the blood stain turning into Louie's signature. Cecilia then does the same by pricking her own finger with a hairpin.
- Assault on Precinct 13 (1976). After several members of their gang are killed in a police ambush, the leaders of the Street Thunder gang stick Sinister Switchblades into their forearms to make the "cholo", a blood oath to avenge them. They eventually choose Precinct 13 as the place to take this revenge.
Wells: It means they don't care. They're not afraid to die, any of them. They want to rip us apart, no matter what it costs. It means to the death.
- Curse of the Crimson Altar: Peter signs Lavinia's book in blood. According to Professor Walsh, this is a step there is no coming back from.
- Fantastic Beasts: The Crimes of Grindelwald reveals that Grindelwald and Dumbledore made a blood pact to never fight each other, which is why Dumbledore has made no moves to directly oppose Grindelwald.
- Faust: Love of the Damned: Jaspers' contract with M for demonic superpowers is, of course, written in blood.
- Allen tries to do this with the rest of the pack in The Hangover. It gets voted down.
- if.... (1968): The three main protagonists Travis, Wallace and Yardley make an oath in this way.
- In John Wick: Chapter 2, the concept of "Markers" is introduced. When made, the participants are given a special medallion that can be imprinted on with two thumbprints in blood; the person asking for the initial favor must place their thumbprint while the helper hangs on the medallion until it comes time to repay the favor. Once the favor has been repaid, the helper adds their thumbprint and also signs a special book at a their nearest Continental to clear the debt. John is revealed to have used one of these to help in his "impossible task" when he initially tried to retire, in this film- the crime lord who he asked for help decides that it's time for John to repay the favor, kickstarting the film's central events.
- The wild men swear to obey Saruman this way in The Lord of the Rings.
- In Mongol young Temudgin (the future Genghis Khan) makes a blood oath with Jamukha, who saved his life. They become blood brothers, complete with slicing their palms and letting the blood drip into a bowl of milk that they drink. In the end, Temudgin lets Jamukha go, even though he proves to be a threat in the future, saying that he's not sparing an enemy, he's freeing his brother.
- My Life as a Dog: Ingemar does this with his friend Lilla to symbolize their “marriage.”
- The Nest: Goyita and Alejandro pledge eternal love for each other, when she carves an A on her palm, he carves a G on his, and they clasp hands. This is creepy, because he is 60 and she is 13.
- The Outlaw Josey Wales and Ten Bears take a blood oath to seal the "words of iron" peace treaty between the Comanches and Josey's friends at the Turner Ranch and Santo Rio. It is strongly implied, though unstated, that this also makes Josey and Ten Bears blood brothers.
- Phantom of the Paradise: Anyone "under contract" in this movie is implied to have signed one, and we see Winslow sign one onscreen.
Swan: Ink isn't worth anything to me, Winslow.
- It's later revealed that Swan was offered a Deal with the Devil, who also asked for a contract sealed in blood. Swan notes that the razor he uses to cut himself he was originally planning to use to slit his wrists before Satan intervened. Satan quips that he's damned either way.
- Robin Hood: Prince of Thieves: Kevin Costner's Robin Hood does it when he swears to avenge his father's death.
- Robin Hood (2010): Used to draw a a parallel between the hero and the villain is them both setting the plot in motion with agreements accidentally sealed with blood. When Sir Godfrey meets with King Philip of France and agrees to help him invade England Philip accidentally cuts himself opening an oyster and Godfrey eats the oyster with Philip's blood still on it. When Robin Longstride agrees to return Sir Robert Loxley's sword to his father, he notices a cut on his hand which his friend points out as this trope, though Robin dismisses it.
- Being deemed negro was an important plot point in Show Boat. By sharing a drop of blood, the hero was now deemed negro and could marry his sweetheart without violating the miscegenation laws.
- Tom and Huck do this in The Adventures of Tom Sawyer after witnessing the murder of Dr. Robinson by Injun Joe, vowing to keep it a secret. Notably, they take at least a bit more care with it than in most examples, as Tom insists on using a needle instead of the brass pin Huck offers to eliminate the possibility of blood poisoning by "verdigrease."
- In The Baroque Cycle, the members of the Cabal swear various oaths of their loyalty to each other and specifically to Jack. Van Hoek swears by his right hand. When Jack reminds him "But Cap'n, you are left-handed!", van Hoek points out that he'll need his strong left hand to chop his right off if he ever breaks the oath — and then, as a symbol, he chops his own little finger off and throws it into the fire. (He then faints into the dust, but it's the symbolism that counts with these things.)
- Recycled In Space for the Boojumverse
- In "The Wreck of the Charles Dexter Ward", the protagonist provides both a thumbprint and DNA sample to seal a contract. There's a Deal with the Devil feel to it as she's been given a Too Good to Be True offer that she's not in a position to refuse.
He slid a tablet across the rented desk. Cynthia pressed her thumb against it; a prick and a buzz, and her blood and print sealed the contract.
- Likewise in "Boojum" involving a crew of Space Pirates. A Boojum is a Living Ship that has the ability to eat intruders, so it would have to identify its crew, presumably through DNA.
Just as she had when she took Captain Song’s oath and slit her thumb with a razorblade and dripped her blood on the Lavinia Whateley’s decking so the ship might know her, Black Alice—metaphorically speaking—took a breath and jumped.
- In "The Wreck of the Charles Dexter Ward", the protagonist provides both a thumbprint and DNA sample to seal a contract. There's a Deal with the Devil feel to it as she's been given a Too Good to Be True offer that she's not in a position to refuse.
- In Bravelands, lions make oaths by lightly scratching each other on the throat and swearing to hold their bargain.
- Marie Brennan's Doppelgänger duology features a blood-oath as a plot point in both books. If the oath is broken, the magically-sealed scar will bleed the Hunter (or witch or Cousin in the second book) to death through one wrist. Unique in that the oath binds both parties to their word (presumably; the actual wording states that the contractor binds him/herself to grant the contractee three boons without conditions, so magical compulsion may or may not extend to the contractor).
- The Elemental Trilogy: Occurs a few times throughout the trilogy, particularly in The Burning Sky, when Titus tricks Iolanthe into making one.
- In the Incarnations of Immortality series, when 2 immortal Incarnations sign an agreement, they choose to shed some blood to sign their names. This binds even the Incarnations to the deal. It happens once between two Incarnations Fate and Satan.
- In It, after defeating Pennywise the first time, the Losers Club make a blood oath by cutting their hands with a broken bottle and promise that if Pennywise ever returned, they would reunite and stop him for good.
- Parodied in one of humorist Patrick McManus's short stories about his childhood adventures with his pal Crazy Eddie Muldoon; the duo always talks about sealing their (not terribly vital) secrets with blood oaths, but since that would involve, you know, cutting yourself, they only do it when one of them accidentally damages himself while playing, and they "share" the blood.
- Nightfall (Series): Myra and Armida strike one. It is more binding to Armida, who is a vampire and cannot break a blood oath.
- The Silerian Trilogy: Shallaheen will bind themselves through ritually cutting open their palms. This can be for adoption, marriages, or vowing revenge.
- A Song of Ice and Fire
- New members of the Second Sons (a mercenary company) initially had to sign their contracts with their blood, but nowadays red ink is used instead, because blood turned out to be a not very optimal substitute for ink. When Tyrion Lannister joins the company though, he adds a bit of his own blood to the ink, stating that he likes traditions.
- Stannis Baratheon uses his own blood to sign an accord with the Iron Bank of Braavos, because his ink has frozen. Given what the Iron Bank does to people who don't pay their debts, it has all the connotations of this trope.
- Star Wars: Bloodline: One of the few duties of the largely ceremonial Supreme Governor of Birren is to take an oath to keep items under royal seal secret even unto death. When the childless Lord Mellowyn dies, the Governorship was supposed to go to Princess Leia as she was next in line but she stepped aside in favor of Lady Carise Sindian. Sindian breaks her oath when she finds a music box containing a message from Bail Organa about Leia's biological father. Leia declines to prosecute this violation to the full extent that she could, but does convinces the Elder Houses to strip Sindian of her Governorship and her royal titles.
- In Tamora Pierce's Tortall Universe, anyone who breaks an agreement that they signed with their own blood will die as their blood literally boils in their veins.
- The Bazhir of Song of the Lioness have a lot of blood oaths. Members share blood to induct new people into the tribe, as part of the ceremony of making someone into the Voice of the Tribes, and as an adoption ritual. Alanna, who becomes part of the Bloody Hawk tribe, once sighs, looking at the scars she's already picked up while with the tribe and thinking she's just vain enough to not want to pick up more.
- Used frequently by Aly in the Trickster's Duet when she's turning enemy spies into double agents for the rebels — it's harsh, but it discourages betrayal better than milder options would, so she doesn't let herself be bothered by it.
- Warhammer 40,000: It is revealed that before the events of the Gaunt's Ghosts series, Gaunt and several other officers made a blood oath with Warmaster Slaydo, promising to continue the Crusade and protect Saint Sabbat to the best of their efforts in the event that he died (which he did), and that this is the reason Gaunt is willing to disobey Lugo's orders.
- In the last season of Angel, Angel signs a contract from Wolfram and Hart in his own blood. He learns this when the person offering him the pen stabs it into his hand.
- In Farscape, when John needs Scorpius's help to rescue Aeryn, Scorpius first makes him perform a "Scarran blood vow", which involves both of them cutting their fingers and drinking each others' blood and then their own. John is understandably squicked.
- House of the Dragon has an interesting example. A Valyrian wedding ceremony merges classic wedding vows with a blood oath. When Rhaenyra proposes to Daemon, its with the line, "Let us bind our blood." The ceremony involves Palm Bloodletting, clasping their bleeding hands, and painting marks onto each other's foreheads in blood. Lastly they cut their bottom lips and kiss with bloody mouths.
- Lost in Space episode "The Space Pirate". After the title character takes Will Smith hostage, he decides that he has to tie Will up before going to sleep so Will can't escape. When Will objects, he proposes that Will take the Pirate's Oath instead. This involves pricking a finger on each of their hands, putting their bloody fingers together and Will taking the Oath.
- Played for Laughs in Malcolm in the Middle, when Hal remembers that he made a promise sealed in blood to nine year-old Francis that he will make up for missing his school play by taking him on a motorcycle trip for his twenty-first birthday. A promise Hal originally made just to get his son to stop crying.
- In Merlin (1998), Merlin swears an oath, complete with drawing a small amount of his own blood, to not use his magic except to defeat Queen Mab.
- The Princess Wei Young: Since they're both sworn to serve Xin Er, Jun Tao puts Bai Zhi through a ritual to make them blood sisters that involves cutting their fingers and drinking each other's blood. Bai Zhi is not exactly enthusiastic about it.
- In Squid Game, Gi-hun is strongarmed by his Loan Shark into putting his fingerprint on a document signing away his physical rights. When he asks for ink, he's told to use the bloody nose he was given moments ago and that they'll take his organs if he misses another payment.
- An episode of Star Trek: Deep Space Nine is actually titled "Blood Oath". In it, Dax feels bound to fulfill a vow of vengeance her previous self made with three Klingon friends twenty-five years ago.
- A frequent trope for the Klingons in most Star Trek series. Everything requiring the slightest bit of commitment must be sealed with some blood from the hand (adoptions, contracts, weddings, birthdays, bachelor parties...)
- Manowar signed their contract with Megaforce Records with their own blood.
- In The Bible, the covenants that God makes with His people require the shedding of blood to make them effective and binding. Abraham's covenant with God that he would posess the land of Canaan would be sealed with every male in Abraham's family, along with his male servants, being circumcised. Moses sprinkled the book of the Mosaic Covenant as well as the Israelites with the blood of goats and calves to sign and seal that covenant. Years later, Jesus would shed His blood on the cross to put into effect the New Covenant between God and His people, the Christians.
- During a famous feud with Jerry Lawler in Memphis, legendarily crazy lumberjack wrestler Jos LeDuc famously took a blood oath (pronounced "oat") swearing he would end Jerry Lawler's career. LeDuc ramped up the squick factor several notches by using a double-bladed axe and REALLY CUTTING A SCAR INTO HIS OWN ARM!
- Earthdawn: Earthdawn Gamemaster Pack, "Gamemastering Earthdawn" booklet. Magical blood charms are used to seal bargains: if either person violates the conditions, the magic punishes the violator.
- In Nomine: Mammon's favored way of sealing contracts with mortals is to have them sign with their own blood. This is for practical reasons more so than symbolic — the supernatural link between the blood and its original owner can be used to track down the signee should they try to run.
- Ironsworn: An iron vow is the equivalent of a blood oath. On top of that, Ironsworn can choose to vow with literal blood.
- In the play example for "Swear an Iron Vow," the player character grips her sword so tightly that it cuts her palm. She envisions that "the blood fuels [her] promise."
- With the Swordmaster asset, your character can let their sword's edge draw blood while you Swear an Iron Vow, which gives a mechanical bonus in exchange for Harm.
- The card Sign in Blood from Magic: The Gathering. Apparently it's quite a long document, because it hurts as much as getting mauled by a bear.
- Shadowrun: Starting with 4th Edition, some of the Triad gangs have started swearing blood oaths that are magically binding, resulting in Death of a Thousand Cuts if they violate those oaths (particularly the oath of secrecy).
- Warhammer 40,000: On the Craftworld Saim-Hann, Wild Rider chieftains and their closest kin perform a ritual on the eve of battle where they paint each other's faces with blood and drink from each other's wrists, reinforcing their commitment to stand alongside each other in life and death. This blood-drinking ritual is also used to seal vows; individuals who break such a pact are shamed and exiled from the Craftworld.
- The World of Darkness:
- Changeling: The Lost has many forms of oaths, and your health or life can be put on the line for them. With heavy costs comes heavy rewards — the more you'll lose if you break the oath, the more benefit you can gain from it.
- Vampire: The Requiem has the Invictus (a covenant of feudalist vampires heavy on inherited power) and their Blood Oaths, magical bonds with heavy penalties for violation.
- Subverted in Damn Yankees, where Applegate laughs at the idea of having Joe sign his name in blood to the contract.
- In Goethe's Faust, as well as the 1926 film adaptation, the title character's contract with Mephistopheles is signed with a drop of blood.
'Mephistopheles: Du unterzeichnest dich mit einem Tröpfchen Blut. (You sign it with a drop of blood.)
- In Fire Emblem: Radiant Dawn, a blood pact is a contract that two or more people sign in blood. If the terms of the pact are not upheld, the person's subjects die off—first slowly, then progressively faster hence the plague that struck years ago. They can only be broken by killing at least one person involved in the pact AND destroying the document.
- God of War: "Ares! Destroy my enemies and my life is yours!" Ascension reveals that this was a Blood Oath. Kratos spilled the blood of his enemies, the blood of the innocent, and the blood of his family. When Kratos turned against Ares after that last one, the Furies hounded him to bring him back to Ares' service.
- Okku from Neverwinter Nights 2: Mask of the Betrayer swears in blood to stop the recently emerged spirit-eater (that is, you, the protagonist). If spared, he calls a truce which lasts as long as you agree to suppress your urge to devour spirits and breaks if you meet certain conditions (commiting some certain evil acts in Okku's presence or getting his approval below zero).
- In Thief II: The Metal Age, Enemy Mines seal their alliance this way (blood on earth variant).
- A quest chain in World of Warcraft starts off with you making one of these with a troll, to show that you'll both honor a truce. He honors it, in that he never does anything to harm you, specifically, although you're tricked into doing horrible things.
- In the backstory, the original Orc chieftains on Draenor gave themselves up willingly to the Burning Legion for power. To seal the pact, the chieftains drank the blood of Mannoroth, binding the entire Orc race to him while infusing them with his fell power.
- In Bad Machinery Erin Winters demands that contracts acknowledging people owe her a favor be signed in blood. Given that the last time she was seen in Scary Go Round before reappearing without explanation in Bad Machinery she was in the company of and (technically) married to Aleister Crowley, and trapped in Hell, this may bode ill.
- Rowasu of Juathuur is full of scars because of this trope. He still breaks the last oath he makes.
- In The Order of the Stick, a Blood Oath binds not just the oath-taker, but all of their descendants. And if they don't die trying to fulfill the oath, they're denied entry into the afterlife while their children have a go at it.
- Of course, the specific instance mentioned here (namely, Eugene Greenhilt's oath against Xykon filtering down to his son Roy) is an odd case. The father in question abandoned the oath pretty quickly, only to try to force it on his child when he was getting old. After the child's death (he died trying to fulfil it), he gets into Heaven while the father is stuck in the "waiting room". When the father gets enraged at this, the angel who made the decree chews out the father for being so careless with a blood oath and points out that the child didn't actually have to take up the oath, but doing so out of free will and trying his best to fulfil it makes him a better person than the father.
- In Sluggy Freelance only Gwynn can use the Book of E-Ville to cast spells because her name has appeared on the inside cover written in blood. Right above instructions to, if lost, drop in any mailbox to return it to the rightful owner.
- Vard, Kai and Marya are bound to the Unknown Champion with one for a year in exchange for her help saving the woods near where Kai and Marya live in Unknown Lands.
- Madoc of Wake The Sleepers accepts a contract through blood on a bounty, as his crazy Assassin culture requires.
- Played With in The Weave; freshly murdered and dying fairy queen Rhiannon gets her blood onto the only witness of the murder, Tally, and subsequently can contact Tally in her dreams because she wants to convince her to help avenging her murder, but Tally really only seals the deal when she agrees to help Rhiannon.
- In one episode of The Powerpuff Girls (2016), Bubbles and Buttercup sell Blossom's soul to HIM for a dollar, and this trope is implied when the girls sign the contract that HIM gives them.
Bubbles: I like the red ink.
- A Season 4 Rick and Morty episode has Morty make a contract with a wizard to receive a dragon Anne McCaffrey-style.
Wizard: Just bleed here, and here.
Rick: I'm not cobleeding.
- There are blood vial pendants for sale around the Internet. The idea behind them is for the couple to carry a bit of each other's blood close to their chests as a sign of their bond.
- The 1876 April Uprising, Bulgaria's most significant rebellion against the Ottoman Empire, was declared through the so-called "Blood Letter", which the Rebel Leader Todor Kableshkov signed with the blood of his hometown's assassinated governor and sent to the town where the revolutionary organisation's HQ resided.
- Generally not a good idea in real life legal practice, though, as seen here.