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Blood brother can refer to one of two things: a male related by birth, or two or more men not related by birth who have sworn loyalty to each other. This is in modern times usually done in a ceremony, known as a blood oath, where having each person make a small cut, usually on a finger, hand or the forearm, and then the two cuts are pressed together and bound, the idea being that each person's blood now flows in the other participant's veins. The act may carry a risk due to blood-borne diseases. In modern times, some gangs have also been rumored to make blood pacts. The process usually provides a participant with a heightened symbolic sense of attachment with another participant.
Örvar-Oddr's saga contains a notable account of blood brotherhood. The Norwegian warrior Örvar-Oddr felt a desire to test his fighting skills with the renowned Swedish warrior Hjalmar. Thus Örvar-Oddr sailed to Sweden with five ships and met Hjalmar, who had fifteen ships. Hjalmar could not accept such an uneven balance of strength and sent away ten of his own ships so that the forces would be even. The two warriors fought for two days with a lot of blood-letting and poetry, but it was a draw. Finally, they realized that they were equals and decided to become sworn brothers by letting their blood flow under a strand of turf raised by a spear. Then, the strand of turf was put back during oaths and incantations.
In the mythology of northern Europe, Gunther and Högni became the blood brothers of Sigurd when he married their sister Gudrun; in Wagner's Ring Cycle, the same occurs between Gunther and Wagner's version of Sigurd, Siegfried, which is marked by the "Blood Brotherhood Leitmotiv". Additionally, it is briefly stated in Lokasenna that Odin and Loki were once blood brothers.
Among the Scythians, the covenantors would allow their blood to drip into a cup; the blood was subsequently mixed with wine and drunk by both participants. Every man was limited to having at most three blood brotherhoods at any time, lest his loyalties be distrusted; as a consequence, blood brotherhood was highly sought after and often preceded by a lengthy period of affiliation and friendship (Lucian, Toxaris). 4th-century BC depictions of two Scythian warriors drinking from a single drinking horn (most notably in a gold appliqué from Kul-Oba) have been associated with the Scythian oath of blood brotherhood.
In Asian cultures, the act and ceremony of becoming blood brothers is generally seen as a tribal relationship, that is, to bring about alliance between tribes. It was practiced for this reason most notably among the Mongols and early Chinese.
The blood oath was used in much the same fashion as has already been described in much of Sub-Saharan Africa. The British colonial administrator Lord Lugard is famous for having become blood brothers with numerous African chiefs as part of his political policy while in Africa. A powerful blood brother of his was the Kikuyu chieftain Waiyaki Wa Hinga.
Blood brothers among larger groups were common in ancient Southeastern Europe where, for example, whole companies of soldiers would become one family through the ceremony. It was perhaps most prevalent in the Balkans during the Ottoman era, as it helped the oppressed people to fight the enemy more effectively; blood brotherhoods were common in what is today Albania, Bosnia and Herzegovina, Bulgaria, Croatia, Greece, Montenegro, Serbia, Republic of Macedonia. Christianity also recognized sworn brotherhood in a ceremony (known as Greek: adelphopoiesis, Slavic: pobratimstvo in the Eastern Orthodox churches; known as Latin: ordo ad fratres faciendum in the Roman Catholic church). The tradition of intertwining arms and drinking wine is also believed to be a representation of becoming blood brothers.
Blood brotherhood, highly ritualized and subjected to a strong code, was a common practice in the Caucasus, especially among the mountaineers. Some relics of this tradition survive to this day.
Famous blood brothers
- The chiefs of the seven Hungarian tribes formed an alliance by drinking from each others blood, while choosing Álmos as their leader in the 9th century AD. 
- Yesükhei and Toghril. Yesükhei was Genghis Khan's father. Toghril is better known by his Chinese name Wang Khan.
- The Greek Nikolaos Kriezotis and the Serbian Vasos Mavrovouniotis in the Greek War of Independence
- Temüjin (Genghis Khan) and Jamukha were childhood friends and blood brothers, although Jamukha later betrayed Temüjin, and was executed at the order of Temüjin.
- Two Norman knights who took part in the Conquest of Britain in 1066, Robert d'Ouilly and Roger d'Ivry, were well known as blood brothers. It was said that they had agreed beforehand to share the profits of this adventure. They both survived Hastings and were granted lands in Oxfordshire and elsewhere, then worked together on various projects such as Wallingford Castle.
- People of the Serbian Revolution (1804–17): Rebel leader Karađorđe (1762–1817) and commander Milutin Savić (1762–1842); Karađorđe and Greek volunteer Giorgakis Olympios (1772–1821); commander Hajduk-Veljko (1780–1813) and Giorgakis Olympios; commanders Stojan Čupić (1765–1815) and Bakal-Milosav; commanders Cincar-Janko (1779–1833), Miloš Pocerac (1776–1811) and Anta Bogićević (1758–1813).
- People of the Principality of Serbia: Prince Milan Obrenović (1854–1901) and Milan Piroćanac (1837–1897); Aćim Čumić (1836–1901) and Kosta Protić (1831–1892); Đura Jakšić (1832–1878) and Stevan Vladislav Kaćanski (1829–1890).
- Samoan professional wrestler "High Chief" Peter Maivia was considered a blood brother of Amituanai Anoa'i, the father of the fellow professional wrestlers Afa and Sika Anoa'i, who are better known as the Wild Samoans, thus the Anoa'i family regard the Maivia line from him on forward as an extension of their own clan.
Folklore and popular culture
- The Norse gods Loki and Odin
- Liu Bei, Guan Yu and Zhang Fei. In the historical novel Romance of the Three Kingdoms by Luo Guanzhong, these three men swore in their famous Oath of the Peach Garden that despite not being born on the same day, their sworn brotherhood would end with them dying on the same day. Histories only mention that the three men were "close like brothers".
- In the Chinese tale Journey to the West, Sun Wukong (the Monkey King) became blood brothers with Niu Mowang (the Bull Demon King), but later on, this brother relationship was forgotten because of a conflict that occurred involving the bull demon's son that caused other problems for Wukong.
- Lost Boy Aidan and Lost Boy Andi [clarification needed]
- In Serbian epic poetry, there are several blood brotherhoods. Miloš Obilić with Milan Toplica and Ivan Kosančić, Miloš Obilić with Prince Marko, Miloš Obilić with the Jugović brothers, Despot Vuk Grgurević and Dmitar Jakšić.
- In the musical Blood Brothers Michael Johnstone and Edward Lyons are blood brothers and stand by each other. They don't realize until the day they die that they are twins.
- In the film Mickybo and Me, the two main characters become blood brothers.
- The characters Snake Eyes and Storm Shadow within the G.I. Joe franchise are portrayed as "blood brothers" as they shared the same master; despite the fact that they are now in fact enemies.
- In the manga franchise One Piece, Luffy, Ace, and Sabo became blood brothers as children by exchanging cups of sake.
- Comedians, 'internetainers' and lifelong friends Rhett and Link have made reference to being blood brothers numerous times throughout their internet videos.
- In the film The Untouchables, Eliot Ness (Kevin Costner) and Jim Malone (Sean Connery) take a blood oath as they work to take down Al Capone (Robert DiNiro).
- The film The Warlords follows a trio of fictional blood sworn brothers during the Taiping rebellion along with their rise and fall.
- Caspar Meyer, Greco-Scythian Art and the Birth of Eurasia: From Classical Antiquity to Russian Modernity, OUP (2013), 246 (fig. 98b) "Gold relief appliqué showing two Scythians drinking from one drinking horn. From Kul-Oba (Inventory 2, K.12h). Rostoftzeff identified the scene with the Scythain sacred oath described in Herodotus 4.70. Fourth century BC. 5 × 3.7 cm, 28.35 gr."; see also "Internet Encyclopedia of Ukraine", Scythian gold statuette depicting the ritual of brotherhood.
- Anonymus (c. 1200). Gesta Hungarorum (PDF).
- Бошко Стрика (1927). Српске задужбине Фрушкогорски манастири: Fruškogorski manastiri. тисак закладе тискаре "Народних новина". p. 173.
- Douglas Dakin (1973). The Greek Struggle for Independence, 1821–1833. University of California Press. p. 36. ISBN 978-0-520-02342-0.
- Béla K. Király; Gunther Erich Rothenberg (1982). War and Society in East Central Europe: The first Serbian uprising 1804-1813. Brooklyn College Press. p. 382. ISBN 978-0-930888-15-2.
- The Slavonic and East European Review. Jonathan Cape Limited. 1928. p. 183.
- The Revolt of the Serbs Against the Turks: (1804–1813). Cambridge University Press. 2012. p. 112. ISBN 978-1-107-67606-0.
- Dušan Baranin (1977). Milan Obrenović: kralj Srbije. V. Karadžić. p. 67.
- Vukadin Sretenović (1990). Kralj Milan. NIGP "Glas". p. 55.
- Gavrilo Kovijanić (1986). Tragom čitališta u Srbiji. Narodna knjiga. p. 138.
- Nebojša Popov (January 2000). The Road to War in Serbia: Trauma and Catharsis. Central European University Press. pp. 192–. ISBN 978-963-9116-56-6.
- Tanya Popovic (1988). Prince Marko: The Hero of South Slavic Epics. Syracuse University Press. pp. 26–. ISBN 978-0-8156-2444-8.
- Anamaria Dutceac Segesten (16 September 2011). Myth, Identity, and Conflict: A Comparative Analysis of Romanian and Serbian Textbooks. Lexington Books. pp. 208–. ISBN 978-0-7391-4865-5.
- Asmus Soerensen; Marija Kleut (1999). Prilog istoriji razvoja srpskog junačkog pesništva. Zavod za udžbenike i nastavna sredstva. p. 55.
- Blood compact, an ancient ritual of the Philippines