The saga relates that Magnus was the leader of the army of Frisians in the conquest of Rome. The eyes of Pope Leo III were pulled out and he was expelled from Rome by a group of nobles that did not accept his authority. With 700 "discerning" Frisians, Magnus reconquered Rome from these nobles. They entered Rome at night by a small port that had become visible through low water level in the Tiber. Through the Flavian gardens, they went to St. Peter's and when they were discovered there a battle ensued. Magnus flew his banner clearly visible on top of the church, creating confusion. There were no prisoners, and the Frisians were in Saint Peter's Square up to their ankles in blood.
When asked by the emperor, Magnus requested no payment by way of nobility, but instead requested the right to freedom, and free speech forever, for all Frisians. Their courage was rewarded with the Karelsprivilege, Friezenprivilege, Vrijheidsprivilege, libertas Frisonica or Previlegii Frisiorum Caroli Magni.
According to the sage the Friesians elected Magnus, that is all Friesian freemen both the born and the unborn, so long as the wind blows the clouds and the world lasts. Also Charles gave them a golden crown in the shield, and a half eagle in their coat of arms, as a token that they had received freedom from the Emperor.
According to one tradition, Magnus was the founder of Church of Santi Michele e Magno, the Friezenkerk in Rome. His father was Gustavus Forteman who possibly founded the first Christian church in 777 in Friesland, the Cathedral Almenum. Magnus was killed in the fight against the Saracens and then became sacrosanct.
Generally most see this story as a legend and the Karelsprivilege as a forgery, although the charter by various kings and even by several popes has been confirmed. Also Jancko Douwama mentions the conquest of Rome by Magnus, the risk and reward in the form of the Karelsprivilege in his "Book of the Parties" in 1525.
- M. Kist en H. Kist and H. Wind, Een man van eer, Bloemlezing uit Jancko Douwama's geschriften, Uitgeverij Verloren, Hilversum, 2003 (a man of honour, Anthology of Jancko Douwama's writings)
- Theun de Vries, Fryske Sêgen III, Leeuwarder Courant 15 January 1927, volume 3 (Enigszinds romantiseerd)
- F. Binkes, Over eene volkplanting der Friesen in Zwitserland, beneevens eenige aanmerkingen over den tocht der Friesen naar Rome, In: De Vrije Fries, episode I (1839), blz. Binkes, Over eene volkplanting der Friesen in Switzerland, beneevens every criticisms over den der Friesen trip to Rome, In: The Free Frisian, I episode (1839), p. 1 t/m 63. 1 to 63. Also available digitally .
- MP v. MP V. Buijtenen, De grondslag van de Friesche Vrijheid (Assen 1953) (16) Buijtenen, (The basis of the Friesian Liberty) (Assen 1953) (16)
- T. van der Laars, Wapens, vlaggen en zegels van Nederland (weapons, flags and seals in the Netherlands), Amsterdam (1913), reprinted 1989, page 58
- K. v. V. Richthofen, Untersuchungen II (Berlin 1882), 147-315 Richthofen,
- P. Sipma, Fon alra Fresena fridome (1947)