Owain was the son of the short-lived Emperor of Britain & Gaul, Magnus Maximus
(alias Macsen Wledig). He remained behind in Britain with his mother when Magnus travelled to his death on the continent. His position allowed him, in the late 4th century, to take control of Mid-South Wales, though he would not have styled himself 'King' at the time.
He may have been decurion of the civilian settlement surrounding the fort at
Tamium (modern Cardiff), or perhaps he held some military rank there. His
influence extended across modern Glamorganshire, an area which appears to have then been known as Cernyw (often confused with the original form of Cornwall).
There is an old story told about how Owain died. He and his brother, King Constantine of North Wales, were escorting their mother, St. Elen Luyddog
(of the Host), through the Mountains of Snowdonia when Owain's rear-guard were attacked by an evil giant, named Cidwm, near Nantmor. The two took part in a fierce battle in which they pelted each other with either arrows or huge steel balls. Eventually Owain killed the giant, but was so badly wounded himself that he shot an arrow into the air and asked to be buried where it fell. He died immediately afterward and was buried at what became Bedd Owain. Later rumours suggested that he had been killed at the instigation of his own brother!