https://en.wikipedia.org/wiki/Conan_Meriadoc

Conan Meriadoc is a legendary Briton leader credited with founding Brittany. Versions of his story circulated in both Brittany and Great Britain from at least the early 12th century, and supplanted earlier legends of Brittany's foundation. His story is known in two major versions, which appear in the Welsh text known as The Dream of Macsen Wledig, and in Geoffrey of Monmouth's Historia Regum Britanniae. Both texts associate him with Magnus Maximus , a Roman usurper of the Valentinian dynasty who was widely regarded as having deprived Britain of its defences when he took its legions to claim the imperial throne. Conan's cousin or sister, Saint Elen, is said to have been Macsen Wledic's wife.

c.340 - c.387. Conan Meriadoc / Conanus. It is not clear in source whether these are the same person as above. He appears as King of Vannetais and Duke of Cornwall. He will leave Dumnonia to his eldest son by a lady named Ursula. In 383, Magnus Maximus, military commander in Britain, leaves for his attempt to gain the Roman Empire. He takes Conan with him. He establishes Conan Meriadoc as king in Armorica at the start of his campaign. Sources suggest that Conan is the nephew of Octavius, Maximus' predecessor. The relationship could make Octavius the father of Maximus' wife, as Conan is also her cousin. Conan's son, Gadeon, is mentioned in the Dream of Macsen Wledig .