He was son of Harald IV Gille, king of Norway and his mistress Thora Guttormsdotter (Þóra Guthormsdóttir).
He was killed in the power-struggle against his brother, Inge, in an early stage of the civil war era in Norway.
Their respective guardians joined forces against Sigurd Slembe and his ally, the former king Magnus the Blind.
During the minority of the brothers, Sigurd, Inge and Magnus, the Norwegian nobility cooperated to rule the kingdom and advise the kings.
At the advice of his mother Ingrid and his senior advisor, Gregorius Dagsson, Inge ordered his men to assault the house where Sigurd was residing.
As it turned out, the killing of king Sigurd started the second phase of the Norwegian civil war era, with fighting continuing with only short let-ups until 1208.
According to the sagas, Eystein and Sigurd had plotted to strip Inge of his royal title and divide his share of the kingdom between them.
The three former base at least part of their account on the older saga Hryggjarstykki, which was written some time between 1150 and 1170, and was thus a near-contemporary source.