He first travelled through Thessaly and arrived at the river Echedorus, where he met the giant Cycnus, the son of Ares and Pyrene, who challenged him to single combat. In this encounter Heracles completely vanquished his opponent, who was killed in the contest; but now a mightier adversary appeared on the scene, for the war-god himself came to avenge his son. A terrible struggle ensued, which had lasted some time, when Zeus interfered between the brothers, and put an end to the strife by hurling a thunderbolt between them.
-page 247, Berens