The following video is an excerpt from the movie, Women in Love, based on the novel of the same name by D.H. Lawrence. The sequence is derived from the chapter, "Gladiatorial."
Blood brother can refer to one of two things: Two males related by birth, or two or more men not related by birth, who swear loyalty to one another. This is usually done in a ceremony (Blood Oath) where the blood of each man is mingled together. In simple terms, this is an extension of fraternization.
The Lydia ceremony involved nicking their arms with a sharp object and licking the blood off of each other's arms. The Scyths would allow their blood to drip into a glass where it was mixed with wine and drunk by both participants.
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.
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 or the forearm, and then the two cuts are pressed together. The idea being that each person's blood now flows in the other participant's veins.