Ragnar Lodbrok's saga tells that Eysteinn was the king of Sweden and a good friend of Ragnar Lodbrok. Eysteinn had the most beautiful daughter named Ingeborg. He was also a devout pagan and there were so many blóts at Uppsala that no other place had ever been its equal. The Swedes worshiped a holy cow named Sibilja, and when the Swedes were attacked by enemies they had the cow walk in front of the battle formation. The cow's magic was so great that when she began to bellow the enemies began to fight between themselves.
When Ragnar Lodbrok was visiting Eysteinn at Uppsala, Eysteinn suggested that Ragnar marry Ingeborg and have her as wife instead of the pauper Aslaug (Kraka). Ragnar consequently betrothed himself to Ingeborg. Eventually, Aslaug found out from three small birds, and told Ragnar that she was no pauper. She was the daughter of Sigurd and Brynhild. As Ragnar understood that Kraka was of better descent that Ingeborg, he decided not to go back to Sweden and wed Eysteinn's daughter.
Eysteinn was upset with Ragnar's change of plans and declared that their friendship was no more. This was interesting news to Ragnar's sons Eric and Agnar who decided to go to Sweden and pillage, as their father would not mind anymore.
Eysteinn sent the fiery cross in all directions and assembled the Swedish leidang. They also made Sibilja join the army, but they had to blót considerably to make her obey. When they had arrived at the location where Eric and Agnar were camped, Eysteinn ordered one third of the Swedish army to attack Eric and Ragnar's men, while the others would attack from all directions after the battle had begun. This was done and Sibilja began to bellow. Eric and Agnar's warriors began to fight between themselves and eventually Agnar died and Eric was taken captive. Eric was offered both peace and Ingeborg but Eric wanted the Swedes to stick spears into the ground and throw him on top of them so that he would die.
When Aslaug heard the news of Eric and Agnar's death, she cried blood, and asked Ragnar's sons to avenge their dead brothers. Ivar the Boneless was afraid of the magic that ruled in Sweden, but when his little brother, the only three year old Sigurd Snake-in-the-Eye wanted to attack Eysteinn, the brothers changed their minds.
Sigurd's foster-father assembled five longships for him. Hvitserk and Björn Ironside mustered fourteen, while Aslaug and Ivar the Boneless marshalled ten ships each. Ivar would not allow his mother Aslaug to go by sea, but she could join the army of riders that would go across land. She accepted and changed her name to Randalin.
When the armies met in battle, Ivar told everyone to make such a din that the bellowing from Sibilja could not be heard. This was of no use, however, and so Ivar shot two arrows in each of Sibilja's eyes. However, the cow could not be stopped. Then Ivar asked his men to throw him on top of the cow. Ivar made himself so heavy as he landed on the cow, that she was crushed.
The Swedish leidang was beaten and Eysteinn fell. Ragnar's sons commanded that their brother had been avenged and that the Swedes should be spared pillaging.
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