King Sigfred is first mentioned in 777 when the Saxon chief Widukind, leader of the resistance against Charlemagne, fled Saxony in the face of the Frankish onslaught.

Shortly after, Widukind left his Nordic refuge and returned to Saxony where he successfully egged the population to rebel against the Frankish king.

If he dared molest Peter and his entourage and refuse Christianisation, he would quickly be led before Charlemagne's throne in fetters, abandoned by Odin (Waten) and Thor (Thonar).

Modern scholarship is in agreement that part of the genealogy of the early Viking Age kings of Denmark is of High medieval era documentation.

[10] The recurrence of the names Angantyr, Sigfred, Harald and Gudfred throughout the 8th and 9th centuries suggests that the kings of the early Viking Age were mutually related.