Geoffrey's largely legendary "Historia Regum Britanniae" expands upon Henry's brief mention, listing Coel as a King of the Britons following the reign of King Asclepiodotus. In the "Historia", Coel grows upset with Asclepiodotus's handling of the Diocletianic Persecution and begins a rebellion in his duchy of Caer Colun (Colchester). He meets Asclepiodotus in battle and kills him, thus taking the kingship of Britain upon himself. Rome, apparently, is pleased that Britain has a new king, and sends senator Constantius Chlorus to negotiate with him. Afraid of the Romans, Coel meets Constantius and agrees to pay tribute and submit to Roman laws as long as he is allowed to retain the kingship. Constantius agrees to these terms, but Coel dies one month later. Constantius marries Coel's daughter, Helena, and crowns himself as Coel's successor. Helena subsequently gives birth to a son who becomes the Emperor Constantine the Great, giving a British pedigree to the Roman imperial line.