Fóstbrðra saga, or The Saga of the Sworn Brothers, is an Icelandic saga from the late 13th century.
About a decade after Iceland has converted to Christianity, best friends Thorgeir Havarson and Thormod Bersason grow up together in the Icelandic Westfjords. Teachings of love and forgiveness are, alas! all wasted on Thorgeir and Thormod, who feel they are not cut out for a pacifist lifestyle, and intend to shape their lives in the ways of the vikings of old. As they believe it is their destiny to die fighting, the two make a pact that whoever of them lives longer will avenge the other, and seal the deal by performing the rites of fóstbrðralag, sworn brotherhood.
Naturally, there comes a time when the fearsome warrior Thorgeir gets himself killed, leaving the scrawny poet Thormod with the duty to avenge his death.
Saga of the Sworn Brothers survives in several manuscripts versions, among which the one found in Flateyjarbók (an illustrated codex from c. 1390) stands out for numerous instances of an oddball black humour not found in the other manuscripts, and also for depicting Thorgeir as a psychopath with a very nonchalant attitude towards homicide. In the other variants, the character of Thorgeir is less extreme, although it appears Flateyjarbók preserves, in fact, the most original version of the saga.
Saga of the Sworn Brothers was retold by Halldór Laxness in his historical novel Gerpla (1952; published in English as The Happy Warriors  and Wayward Heroes ), which however takes major liberties with the plot of the saga.
- An Axe to Grind: The saga takes its time to describe the weapons of the young Thorgeir Havarson, of which the foremost is "a broad axe, a mighty weapon, keen-edged and sharp, with which he had sent many a man to dine in Valhalla" (i.e. killed them). The saga also somewhat apologetically comments that "[i]n those days, very few men were armed with swords". The poetical description of Thorgeir's weapons is undermined by the fact that at this point, Thorgeir is still a teenager who has not yet killed anyone, much less sent them "to dine in Valhalla" (a place that neither the author nor the audience of the saga believed in). This suggests the entire passage is tongue-in-cheek, and that likely teenage Thorgeir simply cannot afford a sword, and that his "mighty weapon" is actually a completely ordinary battle-axe.
- The Berserker: In the Battle of Stiklestad, Thormod (who has already made clear that he intends not to survive King Olaf) always fights in the front line without armour or shield, instead wielding a large axe with both hands, with which "he hack[s] his way through the ranks of the enemy". His behavior earns him "great acclaim for his bravery". Nevertheless he is still unwounded when King Olaf has fallen, much to his own regret. Flateyjarbók furthermore adds that Thormod folds up his tunic under his belt in the front, but lets it hang low in the back because he intends to only go forward and never retreat; that "so many men fell by Thormod's hand that naming them all would take far too long"; and that he stays unwounded because the enemies are scared to face him.
- Come Out, Come Out, Wherever You Are: Having killed Thorgrim and three sons of Thorgrim's sister Thordis, Thormod is planning to leave Greenland, but Thordis gets wind that Thormod is rowing across a fjord near their farm at night and sets out with her last son and five farmhands in order to kill him. When Thormod gets aware of the pursuers, he capsizes his boat and swims to a low-lying islet nearby, digs himself a hole between two rocks and covers himself with sea-weed. Thordis guesses correctly what Thormod has done and orders her posse to search the island and make sure to stab around in the sea-weed with their spears. When they do not find him, Thordis shouts that if Thormod can hear her, he must come out to prove he is truly manly and not a coward ("if he has a man's heart and not a mare's"). Thormod keeps lying still, and the searchers leave without having found him.
- Death Seeker: Before the Battle of Stiklestad, Thormod tells King Olaf that he would rather die than outlive Olaf, in a way that suggests he actually expects Olaf to lose the battle. In the battle, Thormod always fights in the front line without wearing armor or using a shield; despite this, Thormod is still unwounded when King Olaf has been killed and most of his remaining warriors retreat. As he laments that he has not made good on his promise to die with the king, he is struck in the chest by an arrow, and "is glad of the wound because he knew it would prove fatal". Thormod dies from the arrow a short while later. In Flateyjarbók, Thormod even helps a troop of survivors to escape from the battlefield, but refuses to go with them himself, and instead prays to Olaf (whom he already assumes to be a saint) to not let him get away from the battlefield alive. Immediately afterwards he is struck by the fatal arrow.
- In the Hood: When Thormod and Skuf go to the ship ready to sail for Greenland, they meet a stranger who calls himself Gest and is "wearing a hood that prevented them from seeing his face", making it obvious he is a man who does not want to be recognized. Much later, it is revealed that "Gest" is Steinar, a relative of Thorgeir who (like Thormod) wants to go to Greenland in order to avenge Thorgeir.
- The Southpaw: Thormod, a lefthander and "not a strong man", is able to kill the great warrior Thorgrim Troll with an axe borne in his left hand, and afterwards makes a poem about the deed which specifically mentions it was done left-handed. This suggests Thormod credits success to his left-handedness, because Thorgrim did not expect an attack from a man carrying a weapon in his left hand.
- We Have to Get the Bullet Out: (Flateyjárbok version only) After the Battle of Stiklestad, a healing-woman tries to pull out the arrow-head from Thormod's chest with a pair of tongs, but fails because the wound is swollen and the arrow has barbs, and so Thormod does it himself. He pulls out some of the "nerves of his heart" with it, "some of which were red and others white, yellow and green". Looking at the arrowhead, Thormod remarks that King Olaf has fed him well because there's fat in his body ("The roots of this man's heart are white"); moments later, he keels over dead. The procedure is justified insofar as the only reason for it seems to be that Thormod wants to give one more proof of his hardiness, as the saga asserts that he already knows he will die.