The Blood-Brother Ritual (Patreon)

This came up in relation to Frozen of all things, and it's come up before (for example in talking about the middle of The Saga of the Volsungs), but in The Saga of Gísli Súrsson we have a detailed account of four men (two brothers and two of their brothers-in-law) swearing an oath of blood-brotherhood. Here is that ceremony as related in ch. 6, as I'd translate it:

They now went out onto Eyrarhválsoddi [this place name means ‘point of land on a hill on a riverbank or beach’] and they cut an arch out of the ground so that each end was in the earth, and in the middle they stood a richly inlaid spear of such a height that a man could just touch his hand to the nail that fastened the point to the shaft. Now the four men were to go under it: Thorgrím, Gísli, Thorkel, and Véstein. And now they bled themselves and let the blood run together in the earth that was dug out from under the arch and they stirred together all the earth and the blood. And then they all fell to their knees and swore an oath that each would avenge the other like a brother, and they named all the gods as witnesses. And when they all took each other’s hands, Thorgrím said: “I’ll have enough trouble doing this with the two of them, Thorkel and Gísli, my brothers-in-law, but I’m not related at all to Véstein,” and he withdrew his hand.