Nennius

Nennius — or Nemnius or Nemnivus — was a Welsh monk of the 9th century. He has traditionally been attributed with the authorship of the Historia Brittonum, based on the prologue affixed to that work. This attribution is widely considered a secondary (10th century) tradition. 

Nennius was a student of Elvodugus, commonly identified with the bishop Elfodd of Bangor who convinced British ecclesiastics to accept the Continental dating for Easter, and who died in 809 according to the Annales Cambriae. 

Nennius is believed to have lived in the area made up by Brecknockshire and Radnorshire in present-day Powys, Wales. Thus, he lived outside the Anglo-Saxon kingdoms, isolated by mountains in a rural society. Because of the lack of evidence concerning the life of Nennius he has become the subject of legend himself. Welsh traditions include Nennius with Elbodug and others said to have escaped the massacre of Welsh monks by Ethelfrid in 613, fleeing to the north.