Styrbjörn the Strong (Old Norse Styrbjörn Sterki; died about 985) according to late Norse sagas was a son of the Swedish king Olof, and a nephew of Olof's co-ruler and successor Eric the Victorious, who defeated and killed Styrbjörn at the Battle of Fyrisvellir. As with many figures in the sagas, doubts have been cast on his existence, but he is mentioned in a roughly contemporaneous skaldic poem about the battle. According to legend, his original name was Björn, and Styr-, which was added when he had grown up, was an epithet meaning that he was restless, controversially forceful and violent.
It is believed that there once was a full saga about Styrbjörn, but most of what is extant is found in the short Styrbjarnar þáttr Svíakappa. Parts of his story are also retold in Eyrbyggja saga, Saxo Grammaticus' Gesta Danorum (book 10), Knýtlinga saga and Hervarar saga. He is also mentioned in the Heimskringla (several times), and in Yngvars saga víðförla, where Ingvar the Far-Travelled is compared to his kinsman Styrbjörn. Oddr Snorrason also mentions him in Óláfs saga Tryggvasonar (around 1190), writing that Styrbjörn was defeated through magic. In modern days, he is also the hero of a novel called Styrbiorn the Strong by English author Eric Rücker Eddison (1926), and he is featured in The Long Ships, by Frans G Bengtsson.
Styrbjarnar þáttr Svíakappa
Styrbjarnar þáttr Svíakappa ('the tale of Styrbjörn the Swedish Champion'), preserved in the Flatey Book, is the source that contains the most material about Styrbjörn.
According to the tale, Styrbjörn, who was originally called Björn, was the son of Olof, a brother of King Eric, who died of poisoning when Björn was still a young boy. When he was 12 years old, he asked his uncle King Eric for his birthright, but was denied the co-rulership until he turned 16. One day he got into a fight with and killed a courtier, who had hit him on the nose with a drinking horn.
When he was 16, the Thing decided that he was not fit to be king, and instead appointed a man of low birth. His uncle Eric did not want him to stay at home, because of his violent nature and the complaints from the free farmers, so he gave Björn 60 well equipped longships, whereupon the frustrated boy took his sister Gyrid and left. Eric also called him "Styrbjörn", adding Styr- because of his nephew's unruly and quarrelsome nature.
He ravaged the shores of the Baltic Sea and when he was twenty, took the stronghold of Jomsborg from its founder Palnetoke and became the ruler of the Jomsvikings. After some time he allied himself with the Danish king Harald Bluetooth and had his sister Gyrid married to him. Styrbjörn married Harald's daughter Tyra Haraldsdotter whom he was given by Harald for conquering Jomsborg.
Harald gave him even more warriors and now Styrbjörn set about trying to take the throne of Sweden. He sailed with a huge force which included 200 Danish longships in addition to his own Jomsvikings. When they arrived at Föret (Old Norse Fyris) in Uppland, he burnt the ships in order to force his men to fight to the end. However, the Danish force changed its mind and returned to Denmark. Styrbjörn marched alone with his Jomsvikings to Gamla Uppsala. His uncle, however, was prepared and had sent for reinforcements from all directions.
During the first two days, the battle was even. The latter evening, Eric went to the statue of Odin at the Temple at Uppsala, where he made a sacrifice. He promised Odin that if he won the battle, he would belong to Odin and arrive at Valhalla ten years from then. The next day, Eric threw his spear over the enemy and said, "I sacrifice you all to Odin". Styrbjörn and his sworn men stayed and died.