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 each person makes 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. The process usually provides a participant with a heightened symbolic sense of attachment with another participant.
The Norsemen entering into the pact of foster brotherhood (Icelandic: Fóstbræðralag) involved a rite whereby they let their blood flow while they ducked underneath an arch formed by a strip of turf propped up by a spear or spears. An example is described in Gísla saga. In Fóstbræðra saga, the bond of Thorgeir Havarsson (Þorgeir Hávarsson) and Thormod Bersason (Þormóð Bersason) is sealed by such ritual as well, the ritual being called a leikr.
Örvar-Oddr's saga contains another notable account of blood brotherhood. Örvar-Oddr, after fighting the renowned Swedish warrior Hjalmar to a draw, entered into foster-brotherhood with him by this turf-raising ritual. Afterwards, the strand of turf was put back during oaths and incantations.[dubious – discuss]
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.
The Hungarian hajduks had a similar ceremony, though the wine was often replaced with milk so that the blood would be more visible.
In Romance of the Three Kingdoms, the Chinese classical literature, the three main characters took an oath of blood brother, the Oath of the Peach Garden, by sacrificing a black ox and a white horse and swearing faith; other blood oaths involving animal sacrifice were characteristic of rebel groups, such as the uprising led by Deng Maoqi in the 1440s, of criminal organizations, such as the triads or the pirates of Lin Daoqian, and of non-Han ethnic minorities such as the Mongols or Manchu. In Mongolian history, Genghis Khan the Great had an anda, blood brother in Mongolian.
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 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.
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.
In the Chinese TV adaptation The Legend of the Condor Heroes of Jin Yong's novel, the protagonist Guo Jing is blood brothers with the antagonist Yang Kang. He is also blood brothers with the elder Zhou Botong and Genghis Khan's son Tolui.
In Jin Yong's novel The Return of the Condor Heroes as well as its Chinese TV adaptation, the main character Yang Guo is blood brothers with Yelu Qi. He and Cheng Ying and Lu Wushuang form a blood sisters pact, both of them being girls.
^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.