A son of King Morvid of Britain.
He joined his brother Peredur in deposing his elder brother, King Elidur. Iugenius became king of the southern half of Britain. He died after seven years and was succeeded by Peredur.
Iugenius’s son, Idwallo, eventually became king himself.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138