Blood brother can refer to one of two things: two males related by birth, or two or more men not related by birth who have sworn loyalty to each other. This is usually done in a ceremony, known as a blood oath, where the blood of each man is mingled together. 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.
The Lydian ceremony involved nicking their arms with a sharp object and licking the blood off of each other's arms.
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). The Romanian haiducs had a similar ceremony, though the wine was often replaced with milk so that the blood would be more visible.
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. There is some evidence that Native Americans also did it for this purpose.
Blood brothers among large groups was common in ancient Mediterranean Europe where, for example, whole companies of Greek soldiers would become as one family. It was perhaps most prevalent in the Balkan Peninsula during the Ottoman era, as it helped the oppressed people to more effectively fight the enemy. Blood brothers were also common in Serbia, Albania and Bulgaria. Christianity also recognized sworn brotherhood in a ceremony known as adelphopoiesis (in the Eastern Orthodox church) or ordo ad fratres faciendum (in the Roman Catholic church).
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.
It is still practiced today, but mostly as a throw-back to tribal times. The tradition of intertwining arms and drinking wine in Greece and elsewhere, is believed to be a representation of becoming blood brothers.
In modern times, a common blood brother ceremony includes 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. However, such an act is considered highly inadvisable from a medical standpoint due to the risk of blood-borne diseases.
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 in Lokasenna that Odin and Loki were once blood brothers.