Hercules set his foot on the wild lands of Thrace. His cousin, King Eurystheus, had sent him there to seize the four man-eating horses of a local King called Diomedes. After he had captured them, he had to carry them south to Mycenae. Diomedes’s people, the Bistones, were a warlike tribe and the hero was sure that this undertaking might lead to a rather large conflict. Because of that, in this particular Labor, Eurystheus allowed the hero to be escorted by a band of helpers. So Hercules gathered a group of volunteers and set forth to meet the inhospitable and savage denizens of the north. There was a young man who stood out among his followers: Abderos, a son of Hermes, who gained the hero's trust and appreciation thanks to his bravery and eagerness.
Upon their arrival, the band infiltrated the city of the Bistons without trouble and located the stables where Diomedes kept his horses, which he fed every day with human flesh. The usual victims were either slaves or people he condemned to capital punishment, a judgment he very often made. Late in the night, the hero's group attacked the unaware stable guards and captured the wild mares. They were extremely dangerous and Hercules had to carry them away by himself, holding them by iron chains, to ensure his comrades' safety. The band managed to escape the city but the horses' hooves and cries betrayed the path they followed.
Soon the Bistones, led by King Diomedes himself, set on the group’s trail. Before long, their pursuers reached them and Hercules had to abandon the horses to join the fight. Abderos offered to hold the reigns in his stead and Hercules reluctantly agreed. Then the hero urged to the battle that had already started among the Bistones and his men. Hercules bested Diomedes in single combat and when their King had fallen, the rest of the Bistones fled towards their city, intimidated.
Hercules then returned to the mares and was shocked to discover that Abderus was lying dead, killed by the beasts dragging him all over the rocky soil. Enraged, Hercules fed the raging beasts with their own master's body and the horses calmed after that meal. At the place where Abderus had fallen, Hercules founded the city of Abdera in honor of his dead friend and left for Mycenae, the horses’ reins in his hands. Thus his fifth Labor was done.
While wandering in the lands of Morpheus, Hercules was visited by Hermes. "My son was too young to handle those mares, for these man-eating beasts have power beyond imagining. Like violent thoughts that penetrate the barrier of reality and become a living nightmare, those beasts had to be fed with their master's flesh to calm down. When hunting your wayward thoughts and impulses, Hercules, remember: you have to face the disarranged mind that produces them."