Mistilteinn first belonged to Þráinn, who had been king in Valland before he retired in his burial mound with his wealth.
The Danish king Óláfr and his men, among whom Hrómundr Gripsson, learnt about that and found the barrow. Þráinn, who had become a draugr (living dead) was sitting inside. No one but Hrómundr dared to enter. After a long and fierce fight, he defeated Þráinn and took his treasure, especially his sword, with which Þráinn had killed four hundred and twenty men, including the Swedish king Semingr.
Hrómundr used Mistilteinn during the battle between Óláfr and two Swedish kings both named Haldingr. He killed Helgi inn frækni (the Valiant), who had slain his brothers. He then lost Mistilteinn in the water out of witchcraft. He deeply felt this loss but soon recovered his sword, which was found in the stomach of a pike. But Mistilteinn was of no help when he fought king Haldingr, whom he eventually killed with a club.