He ruled the area around Hadrian's Wall when the Romans left Britain.

Magna Charta, Parts I & II, (1945) John S. Wurts, page 160

Prince Coel, son of Cyllin, was living A.D. 120. (Dr. Anderson,
referred to in Wurts, makes him identical with King Coel, son of
Marius, but this is evidently a misapprehension.) Prince Coel was the
father of King Lleuver Mawr the second Blessed Sovereign.
