For him to win Astrid, tha daughter of the Wendish leader Burislav, he swore to free the Wends of the thrallgeld they had to betell to the Danes. He fulfilled his oath by sailing to Sealland where he sent his errand to Sweyn Forkbeard that he had weighty tidings, but had fallen sick and could not come himself to bring them to him. As Sweyn was asksome, he went aboard Sigvaldi's ship and was kidnapped by the Jomsvikings. To be freed the Danish king had to bestow selfhood to both the Jomsvikings and to the Wends, furthermore having to betell a king's letgeld.
At the begraving of his father Strutharald, Sigvaldi was reded by Sweyn to set upon Norway and to outtake Haakon Jarl. This oath led to the Clash of Hjörungavágr in 986, from which Sigvaldi fled with shame.
In 1000, Sigvaldi was shown to be falseful at the Clash of Svolder, by drawing in Olaf Tryggvason to the struggle and by forsaking him in the heat of the clash.