"For he to-day that sheds his blood with me shall be my brother."
— Henry V
Where two people have made a pact with each other. The reasons for this tend to vary, but most often, it's because one or both of them owes the other his life — frequently in battle. From this moment on, they are brothers. Brothers in the sense that they will do anything to protect and, if necessary, avenge the other.
Sometimes this is portrayed with the two taking a Blood Oath, cutting themselves to "mix" their blood. Thus, the trope name.
This trope is very common with Proud Warrior Race Guys, as well as a popular theme in kung fu movies. It's also quite popular in Heroic Bloodshed movies featuring The Triads and the Tongs, who have Blood Brotherhood as a central theme of their organization, best expressed through the 36 Oaths that formal triad members take, which basically boil down to "Do not screw over your sworn brothers, or you will get chopped up by lots of blades." Being criminals, however, Triads and Tongs can be just as prone to backstabbing and betrayal as any other criminal organization, which is especially shown in the grittier cinematic portrayals.
Often, Blood Brothers spend so much time with each other that they're Heterosexual Life Partners. See also True Companions, for when a group of people, because of circumstance, are as close as real family would be.
For the musical, see Blood Brothers. Compare Fire-Forged Friends, Bash Brothers. A Sub Trope to True Companions. Note that all examples must include an oath, pact, or other explicit bonding; if it doesn't, it's not this trope.
While you can certainly declare yourself a "blood brother" to another, actually performing a blood pact (cutting one's hand and all) with that person is not recommended nowadays due to the risk of HIV infection.
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Anime and Manga
In the Fushigi Yuugi series, Genbu Celestial Warriors, Hikitsu and Tomite are the classic pair.
The manga Gorgeous Carat has Ray and Azura who became blood brothers as kids, complete with mixing of blood. Things go downhill in the present when Azura is revealed to be evil, and is the villain for most of the series.
In One Piece, it has been revealed that Luffy, Ace and Sabo were this trope. None of them were related of course, but they were very close and even made a ceremony of drinking sake and became brothers.
Diva's Chevaliers in Blood+ refer to each other as blood brothers since they have all been turned into Chevaliers because of Diva's blood.
Reinhard and Kircheis in Legend of Galactic Heroes are this along with Hetero Sexual Lifepartners. Their friendship started out when Reinhards sister (whom Kircheis is in love with) asks Kircheis to be his friend. Then, Reinhard asks for Kircheis help in conquering the galaxy. And so the two rise up in ranks, Reinhard as the shining star and Kircheis supporting him from behind. And then Kircheis dies (very early on, to boot), something that has lasting effects on virtually everyone for the entire rest of the series.
In Saint Beast, Judas repeatedly asks Luca to stand by him, and when he tries to do things alone Luca reminds him that alone means both of them.
G.I. Joe: Joe has Snake Eyes and Storm Shadow, who are the equivalent of Blood Brothers ("sword-brothers") and have indeed saved each other's life in battle. Sometimes even while actively trying to defeat/kill each other. Heavy on elements of The Only One Allowed to Defeat You, Enemy Mine, and Brainwashed and Crazy, the latter on Storm Shadow's part (at least when they're on opposite sides). Storm Shadow saving Snake Eyes' life is an integral part of their backstory.
There's a pair of Marvel Comics villains known as the Blood Brothers, who are formidable separately but increasingly unstoppable the closer they are to one another..
In a Spider Man/Wolverine crossover by Jason Aaron, the two heroes end up bouncing back and forth through time due to the machinations of X-Men villain Mojo. Toward the end of the series, when they seem to have settled in the Wild West, Logan(who had taken up with local tribe) performed a blood-brother ritual with Peter Parker. Regrettably this seems to have been forgotten after the mini-series ended
Happens sometimes in Knights of the Old Republic fanfics (more for the first than the second) with Canderous and the Player Character...but sometimes all the crew gets marked as vode. The Mandalorian language doesn't make the distinction between "friend" and "family member" after all.
Naruto and Haku become Sworn Brothers in the fanfic A Fathers Wrath swearing an oath in a Peach Garden.
The film version of Othello has Othello forcing Iago to perform a blood pact.
The 2007 Chinese move The Warlords (starring Jet Li, Andy Lau, and Takeshi Kaneshiro/Jin Chengwu) has the three actors' characters playing sworn brothers along the lines of the Peach Tree Oath quoted above, but with tragic consequences since it doesn't hold nearly as well.
Bullet In The Head, Woo's grimmest and most emotionally devastating movie, is about the disastrous effect of Gold Fever and the horrors of The Vietnam War on the bond between Blood Brothers, which in John Woo's other movies was all but unbreakable.
In the 2009 Sherlock Holmes film, a fortune teller reads Watson's palm and refers to "two brothers, not of blood but of bond". While this trope is probably true of them anyway, her actual statement is suspect because Holmes put her up to saying it.
An old Bud Spencer spaghetti-Western, where the (anti)hero, played by Bud Spencer, had a rather dim Native American sidekick, who followed him around due to them being 'Blood brothers', since Bud had once saved his life with a blood transfusion.
According to the liner notes on the Blues Brothers album, Jake and Elwood Blues, unrelated orphans growing up in the same dismal orphanage, swore an oath of Blood Brotherhood by cutting their fingers with an E-string allegedly from Elmore James's guitar, essentially adopting each other. The Mission from God came later. According to the book Blues Brothers: Private, they also had their names legally changed in 1962, taking the same last name and taking each other's first names as their middle names. The Mission from God came later.
In Mongol, two ten year olds (one of whom is the future Genghis Khan) become blood brothers after one saves the other's life.
Interesting twist in Practical Magic, where a pair of sisters swear bloodbrotherhood. This becomes a plot point a couple of times.
In Son of Rambow, Lee Carter decides he and Will Proudfoot should become blood brothers after he saves Will from drowning.
In the Taiwanese gangster film Monga the main character becomes blood brothers with his four best friends in a ceremony that echoes the Peach Tree Oath.
The Outlaw Josey Wales takes a Blood Oath with Comanche chief Ten Bears to seal the peace between Ten Bear's Comanche tribe and Josey's "tribe" of white, Native American, and Hispanic settlers, implicitly making them blood brothers as well.
In Wild Zero, Guitar Wolf cuts Ace's hand and his own to make them "rock 'n' roll blood brothers," and gives Ace a whistle to use if in distress.
In McLintock!, Puma reminds McLintock of their blood-brother bond when they meet at the railroad depot.
In Puss in Boots, Puss and Humpty Dumpty become blood-and-yolk brothers, pricking their fingers with a pin, then shaking hands.
While water doesn't seem all that binding on a planet that is two thirds water, for the Martians it is a very valuable and limited resource and to share it with someone is a precious act of camaraderie.
Mercedes Lackey's Tarma and Kethry are Blood Sisters, complete with a blood-mixing ritual of clasped hands. The pair were given a stamp of divine approval from Tarma's goddess when the wounded hands were instantly healed upon contact, leaving distinctive scars.
Talia eventually does this with both Skif and Kris. Would've been nice if someone had told Dirk that before he went all I Want My Beloved to Be Happy, but then I suppose we wouldn't have the third book.
Mercedes Lackey also uses it in the Tales of the Five Hundred Kingdoms series; in order to stop the Tradition from making her fall in love with her rescuer, Princess Andromeda suggests the idea of becoming blood siblings with George instead. This might have been unnecessary, since George is a woman and it's unlikely the Tradition would make two women fall in love since that doesn't typically happen in fairy tales.
In Romance of the Three Kingdoms, Liu Bei, Guan Yu, and Zhang Fei swear an oath of brotherhood under the tree of a peach garden as quoted above that lasts for their whole lives. They are not the only sworn brothers in the story (for example, Sun Ce and Zhou Yu) but by far the most famous, and for multiple chapters the extent to which Guan Yu will go to uphold this is a plot point, used both by and against him: in particular, Zhang Liao got Guan Yu to surrender to Cao Cao by saying that he shouldn't go out fighting while the missing Liu Bei might still need him), but Guan Yu conditioned that surrender as lasting only until he found his brother's whereabouts... and then openly rubbed the impermanence of this service in Cao Cao's face. Cao Cao however then made Guan Yu the General Who Doesn't Do Anything, up until his opposing warlord's main warrior was too much for Cao Cao's officers to handle and he was forced to pull Guan Yu off from the bench into his lineup.
An Aiel ritual in The Wheel of Time can make you brother and sister, the ritual is shown in detail when Elayne and Aviendha go through it to become First-sisters.
As a more literal example, several Borderlander monarchs go well out of their way to meet in person for a ritual of this sort, treated as much more serious than a written treaty.
In The Bible, David and Jonathan, make a covenant (pact) and their souls are bound together, making this trope Older Than Feudalism. Jonathan will do anything to protect David, namely betraying his own family.
In the Catholic Dogma, our true father is God. Thus, we are ALL Blood Brothers.
Judge Dee's three lieutenants, Ma Joong, Chiao Tai and Tao Gan, consider themselves and refer to each other as Blood Brothers.
In his later Emperor series, Jochi and Jebe do the blood mixing thing.
The Devil In Vienna has a female example, with Jewish Inge and Catholic Lieselotte copying a blood brotherhood ritual-drinking wine mixed with their own blood- from a western novel. They always sign their letter "B.S.L" for "Blood Sisterly Love."
Hagan and Waldhari in Stephan Grundy's Attila's Treasure, especially poignant because of the huge differences between them. As the book progresses it seems as though their devotion to each other only grows stronger the wider the gulf between them.
Horus Heresy: Saul Tarvitz and Nathaniel Garro. Instead of mixing blood, they carved an Imperial eagle into their gauntlets. It even states that there is only one member of the Emperor's Children Garro considers his brother - Tarvitz.
In Rosemary Sutcliff's novel Blood Feud, English lad Jestyn is sold as a thrall to a Dane called Thormod, but after a few battles they realise they are inseparable. They become blood brothers after a blood-mixing pact.
Novels Of The Change: The anamchara ritual has this effect. Unlike most takes on this trope, it does not preclude romance.
In the Xanth novel Centaur Aisle, two allies are wounded during an unbalanced swordfight, and one says, "Now our blood has mingled; we are blood brothers."
In Gisle Surssøns saga, one of the Icelandic lineage sagas, the protagonist Gisle makes a pact with his brother Torkel and his brothers-in-law Vestein and Torgrim. The four of them mix blood by each letting theirs flow into a hole dug into the ground whereupon they cover it with a tussock. In this way, they become sworn brothers who must avenge each other if either of them should be killed. It does not end well when Vestein is murdered and Gisle mistakenly believes that it was by Torgrim.
Winnetou and Old Shatterhand in the books of Karl May.
The title character of the Flashman novels swears blood-brotherhood with several characters, noting in Flashman at the Charge that he felt more solemnity in the act than any Christian oath.
In Alvin Fernald; Superweasel, one of the Alvin Fernald novels by Clifford B. Hicks, pre-teen hero Alvin Fernald, his little sister Daphne and his best friend Shoie create an anti-pollution superheroic identity named "Superweasel". At one point Alvin hits on the idea of a blood oath to seal their heroic pact, but not wanting to hurt his little sister, has them smear drops of iodine on their hands instead of cutting them.
Tarl Cabot becomes a blood brother to various characters a number of times over the course of the Gor series. In particular, Blood Brothers of Gor starts out by playing with the trope with Cabot having been Made a Slave by Canka for having saved the life of Canka's brother Cuwignaka but ends up being Exactly What It Says on the Tin.
Played with in Buffy the Vampire Slayer, season 5. Although Dawn is confused about her own existence as Buffy's sister, an important scene involves Buffy and Dawn clasping bloody hands and Buffy declaring that the same blood, "Summers blood", runs through their veins. So not necessarily making them "blood sisters", but affirming the link that already exists.
In Hercules: The Legendary Journeys, Hercules and Iolaus have admitted to loving each other as brothers. (The fanfic writers often choose to ignore those last two words.)
Kamen Rider Kabuto uses this trope most clearly in the form of the Hopper Brothers: Yaguruma Sou (Kick Hopper) and Kagayama Shun (Punch Hopper).
Pawn Stars: Chumlee and Corey are very much like brothers. Much to Rick's dismay.
Ted declares this for Barney and Marshall in the season three finale of How I Met Your Mother, which causes Barney to start needling him about who's his best brother, in reference to his and Marshall's long running argument over who's Ted's best friend.
Coupling has Jeff and Steve who grew up in school together, and have remained best friends even into their busy and commercial London lifestyle orientated adulthood. So close in fact, that they are in fact porn buddies. By Jeff's wording, when one dies, the other is morally obliged to go to their deceased friend's house and remove all of the porn before their family and colleagues find it. He even boldly defends this pact when talking to Steve's future partner to big his friend up. Disclaiming "that's how close we are!"
On The Andy Griffith Show Opie & a new boy named Trey become blood brothers, only they use barn paint instead because they don't want to get an infection.
In Game of Thrones, Robert Baratheon admits that he has never truly liked his brothers, Stannis and Renly, and that he considers Ned Stark his true brother; the brother he "chose".
Stella does it with her foster sister in CSI NY, in a flashback during the Cold Case crossover ep. The other woman kept some of the blood in a necklace vial, and when she killed someone in the other show's territory, the necklace broke and led the CC guys to New York.
In the song "Goodbye Earl" by the Dixie Chicks, Mary Ann and Wanda are Blood Sisters. When Wanda is put into the hospital by Earl, they get together and kill him.
Odin, Chieftain of the Aesir, and Loki, trickster demigod, were celebrated blood-brothers, and Odin would never sit at a table where Loki wasn't welcome. This causes problems when, in the leadup to Ragnarok, Loki's villainy reaches new heights and Odin can no longer remain blind to it.
Örvar-Oddr and Hjalmar, two heroes from the legendary sagas who became blood brothers in a ceremony after pouring their blood under some raised soil.
The famous Anoa'i family, which pioneered the Wild Samoan archetype, was formed by Amituanai Anoa'i and "High Chief" Peter Maivia, two men who did see one another as blood brothers (Anoa'i's children referred to Maivia as their uncle). The family includes Afa and Sika Anoa'i (the original Wild Samoans) The Rock (Dwayne Johnson), the late Rodney Anoa'i (a.k.a. Yokozuna), Solofa Fatu (Rikishi), The Tonga Kid, and Eddie Fatu (known as YOU MANGA Umaga).
In one of the 90s episodes of Titanes en el Ring, La Momia cut a ribbon, symbolizing the end of his feud with the late Martin Karadagian, and effectively making them Blood Brothers.
All Space Marines ever in Warhammer 40000. They even call each other "Brother", constantly, though this may be as much their Catholic Space Nazism somewhat monastic style as their fanatical loyalty to the Chapter and one another. The Blood Angels co-opt the quote from Henry V :"He who sheds blood with me today shall be my battle brother eternal."
And they have all been sort-of genetically manipulated to be similar to their chapter's Founder, essentially making them his children.
For a slightly more heretical version, teams of Tau Fire Warriors go through the Ta'lissera bonding ritual, producing their own squad-scale True Companions. It also gives them sufficient steel in their little blue backs to return even when eight of the original ten are dead, to avenge their fallen blood kin and kick ass for the Greater Good.
In module CM1 "Test of the Warlords", the PC's can participate in the ritual of blood sharing with a barbarian chief.
Ravenloft supplement Van Richten's Guide to the Vistani. A giorgio (non-Vistani) who performs a great service for the Vistani may be offered the honor of performing the blood rite. A Vistani cuts the left hand of the giorgio and himself and clasps hands with him. After tying a scarf around their hands, he speaks an oath that says they are brothers. When they separate their hands the wounds are healed, leaving a purple scar on the giorgio's hand that identifies him as a giogoto ("enlightened, yet not of the blood") to all Vistani.
Basic D&D Module DA3 City of the Gods gives a chance of sand folk doing this ritual with your party as a means of friendship or adoption. However, their blood is slightly poisonous to the human characters (but only cause minor damage).
1st Edition supplement The Rogues Gallery. The lizard man Phoebus can make another character his "blood kin" by performing a ritual that involves a sharing of blood. If Phoebus or any of his blood kin die, the others will will know that one of them has died and can locate the body of the victim.
Changeling: The Dreaming has the Oath of Clasped Hands, which is your standard blood-mixing ritual with in-game results. (Specifically, both parties get an additional point of Willpower for as long as the oath is upheld.) It's described as the platonic version of the Oath of Truehearts, which is expressly for lovers.
This is also how motleys usually form up in Changeling The Lost. Groups of changelings swear an oath on the Wyrd to come together for a common purpose, and usually gain mechanical benefits related to that area on the condition that they stand up for one another and don't flag in their pursuit. Likewise, there are other Oaths that provide similar benefits, such as the Ancient Pact (which is when a changeling swears one of these with a mortal) and the Vernal Oath (which is a marriage oath).
In 7th Sea, the Eisen have the concept of the rucken — which means "back-brother" and is considered the the strongest oath an Eisen can give outside of marriage. The rucken is someone you can absolutely trust to have your back and vice-versa. Woe to anyone who betrays their rucken's trust and willingly breaks that vow.
Ars Magica supplement The Order of Hermes. When two mages want to form a bond of amicus (intense friendship), they cut their wrists and place their wrists together to share their blood. Grogs sometimes do this too, and specifically call each other "blood brother".
An alien species called Hunters in the Halo series always come in pairs, and will try to avenge the death of the other. This rarely happens though, as they are both practically walking tanks, unless you happen to be the Master Chief.
Hunters (m'galekglo) are actually hive-mind colonies of lekglo worms encased in suits of armor. A bonded pair of Hunters is a colony that got too big for just one suit of armor; so the 'rage' when one dies is more like "That guy just killed a entire city filled with our relatives lets go murder him."
In Homestuck, Karkat Vantas and his universe's version of Jack Noir team up to overthrow the Black Queen, sealing their pact with a bloody handshake. The "blood" part is especially important here, as Karkat sympathized with Jack Noir when he discovered that there was somebody else who had his mutant, "candy-colored" bright red blood.
The Saga Of Tuck: The Euro-mutt title character and his Chinese-American friend, Mike, call each other brother when not trading blows. This isn't just rhetoric: the ritual is re-enacted more than once during the story and allows a kind of psychic link. Tuck claims he can't die unless Mike lets him go, and vice versa. This is so far unproven, but it was close.
Carmilla and Fey, in the Whateley Universe, even going so far as making a blood pact.
In The Gungan Council, Ryori and Ti'Cira went so far as to use blood in their pact to be faithful allies to each other. Darth Apparatus and Chaos Maxtor have a just as binding pact, minus the blood, where Chaos becomes the Darth's soldier while the latter gets functional immortality.
Parodied in The Simpsons when Bart and Milhouse became "spit brothers", complete with spitting in the hands and clasping them. Ewww...
Used in American Dad, when Stan and his new friend become brothers. The next scene shows them both at a hospital for a blood test, happily showing each other the negative test results.
Parodied on The Amazing World of Gumball when Tobias tries to cut his hand open with a blunt stick. It doesn't work very well and he keeps trying to tear the skin well into the next scene.
In South Park, Stan and Kyle can be trusted to consistently treat one another like family, going to considerable lengths for one another in a variety of episodes (for example "Cherokee Hair Tampons", "Super Best Friends", "Imaginationland", "Guitar Queer-o", "Follow That Egg", "The List", "You Have 0 Friends"—you get the idea); Kyle even goes so far as to call Stan his brother in "Ike's Wee-Wee." However, they treat Butters and Kenny more ambivalently, and make no secret of their contempt for Cartman, making the dynamic between the South Park boys more indicative of two Blood Brothers surrounded by many morons.
Temujin, who became Genghis Khan, formed a blood brotherhood with his best friend, Jamuqa, which was common in Mongolia at the time. To seal this, they cut themselves, dripped blood into fermented mare's milk, and drank each other's blood. Keep in mind, they were about 10 or 11 years old when they did this. Because Mongols are badass. Incidentally, Jamuqa ended up as Temujin's main enemy in the wars to unify Mongolia. But when Temujin finally captured him, he offered him a full pardon, but Jamuqa refused, suggesting that Temujin kill him. Temujin took his blood-brother's advice, and killed him, then gave him a proper burial. Because he was nice like that.
To be more specific, they became enemies because Jamuqa had made his own bid to unite the Mongols. While the two had always supported one another, neither could accept the other's rule; Jamuqa compared it to there being only room in the sky for one sun, and only room in the world for one Mongol lord. Jamuqa also requested he be executed without spilling his blood; Mongol religious beliefs held that dying with your blood hitting the ground cursed you to wander the earth as a spirit. Genghis Khan reportedly strangled Jamuqa himself, crying as he did so. As for the generals who had betrayed Jamuqa to Temujin, they were boiled in oil.
A popular item of Filipino history is the Blood Compact (sanduguan in Tagalog, meaning "one blood" or mingling of blood) between Miguel Lopez De Legazpi and the King of Bohol (Datu Sikatuna) in 1565. A contrast to the Filipino revolutionary organization KKK (not that KKK) initiation ceremony where the initiate would cut open an arm vein and use the blood as quill pen ink to write his name on the members' list.
This was done by the leaders of the seven tribes that would eventually become the people of Hungary. The event is referred to as the Blood Pact, an important historical event.
In the Chinese historical saga Romance of the Three Kingdoms, the Power Trio of the Kingdom of Shu faction were blood brothers. Specifically, Liu Bei, Zhang Fei, and Guan Yu aka God Of War.