He is remembered as a King of the Britons and the father of Elen Luyddog and Conan Meriadoc in sources such as the Welsh prose tale The Dream of Macsen Wledig and Geoffrey of Monmouth's Latin chronicle Historia Regum Britanniae.
[3] Geoffrey of Monmouth, in his Historia Regum Britanniae, renders the name in pseudo-Latin form as Octavius and introduces him as a half-brother to Constantine I, who has become King of the Britons following the death of his father Constantius.
Constantine responds by sending three legions to Britain under the command of his great-uncle Trahern, the brother of the late King Coel.
Trahern lands at "Kaerperis" and captures it, forcing Octavius to conscript all the island into the army to combat the Roman legions.
Once arrived, he scatters the Roman forces and retakes the throne of Britain, gaining an incredible amount of wealth and prestige.
Everything he finds is exactly as in his dream, including the presence of her young, gwyddbwyll-playing brothers Cynan (Conan Meriadoc) and Gadeon, as well as her father, king Eudaf Hen, son of Caradawc (Caradog ap Bran).
Because Elen is found a virgin, Macsen gives her father sovereignty over the island of Britain and orders three castles built for his bride.
With the help of men from Britain led by Cynan and Gadeon, Macsen marches across Gaul and Italy and recaptures Rome.
[7] The "daughter of Eudaf Hir (the Tall)" is mentioned briefly in the famous poem Gododdin of Aneirin: The poet Lewis Glyn Cothi (fl.