Magnus Forteman (809), was the legendary first potestaat (or magistrate governor) and commander of Friesland which is now a province of Netherlands. His existence is based on a sage's writings.
According to this saga he was the first of the 17 potestates of Friesland in the period after the Kings of Friesland, a period of Charlemagne until Albert of Saxony (800–1498).
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.