King Ingi married a woman called Mær who had a brother called Svein. King Ingi liked Svein better than any other man, and Svein became thereby the greatest man in Sweden.
However, Inge did not permit the people to follow the old ways, unlike his father Stenkil. The Swedes reacted strongly and asked Inge to either comply with the old traditions or abdicate. When Inge proclaimed that he would not abandon the true faith, the people pelted him with stones and chased him away. This was the opportunity for Sweyn to assume power, and the account provided by Hervarar saga concerning his inauguration contains a rare description of the ancient Indo-European ritual of horse sacrifice:
Svein, the King's brother-in-law, remained behind in the assembly, and offered the Swedes to do sacrifices on their behalf if they would give him the Kingdom. They all agreed to accept Svein's offer, and he was then recognized as King over all Sweden. A horse was then brought to the assembly and hewn in pieces and cut up for eating, and the sacred tree was smeared with blood. Then all the Swedes abandoned Christianity, and sacrifices started again. They drove King Ingi away; and he went into Vestergötland.
This legend is, however, considered to be a later invention by the Church as more reliable sources date Eskil's death to c. 1016, several generations before Blot-Sweyn.
Svein the Sacrificer was King of Sweden for three years. King Ingi set off with his retinue and some of his followers, though it was but as small force. He then rode eastwards by Småland and into Östergötland and then into Sweden. He rode both day and night, and came upon Svein suddenly in the early morning. They caught him in his house and set it on fire and burned the band of men who were within. There was a baron called Thjof who was burnt inside. He had been previously in the retinue of Svein the Sacrificer. Svein himself left the house, but was slain immediately.
A similar account also appears in the Orkneyinga saga, but in this account, Sweyn stays indoors and is burnt to death:
Christianity was then young in Sweden; there were then many men who went about with witchcraft, and thought by that to become wise and knowing of many things which had not yet come to pass. King Ingi was a thorough Christian man, and all wizards were loathsome to him. He took great pains to root out those evil ways which had long gone hand in hand with heathendom, but the rulers of the land and the great freeholders took it ill that their bad customs were found fault with. So it came about that the freemen chose them another king, Sweyn, the queen’s brother, who still held to his sacrifices to idols, and was called Sacrifice-Sweyn. Before him king Ingi was forced to fly the land into West-Gothland; but the end of their dealings was, that king Ingi took the house over Sweyn’s head and burnt him inside it. After that he took all the land under him. Then he still went on rooting out many bad ways.
At that time there were many people all around in the Swedish dominions who were heathens, and many were bad Christians; for there were some of the kings who renounced Christianity, and continued heathen sacrifices, as Blotsvein, and afterwards Eirik Arsale, had done.
Blot-Sweyn is believed to have been the father of Eric of Good Harvests (Eirik Arsale). This Eric is mentioned by a plausible source as the father of Sverker the Elder, and so Blot-Sweyn could be the progenitor of the House of Sverker.