Sibilja was a sacred war cow, much revered in Sweden. She possessed a destructive bellow capable of turning the minds of any attacking force to madness. Agnar & Eric Ragnarsson had already found this out after making war on King Eystein in their father's absence.
Sibilja made short work of their men and soon many fled or lashed out at their fellows in fury and confusion. The Ragnarsson's kept their minds, being of such stout kin, but Agnar was slain by the horns of Sibilja and Eric was captured and taken prisoner. Despite being offered peace and the daughter of King Eystein in marriage, Eric elected to instead be killed on spears and raised above the battlefield so he would not be overlooked for Valhalla after such a defeat.
Hearing of their brother's death the surviving sons of Ragnar and their mother Aslaug made ready for war. It was Ivar the Boneless, the largest and strongest Son of Ragnar who led the next attack and managed to subdue Sibilja himself, shooting arrows through her eyes, blinding her before wrestling her down, slitting her throat and silencing her bellows for good, avenging his brothers.