Diomedes, King of Thrace, was a son of Ares and Cyrene. He lived on the shores of the Black Sea.
The eighth labour of Heracles was to steal the Mares of Diomedes. There were four mares, kept tethered to a bronze manger because they were wild, man-eating and uncontrollable.
When Heracles arrived, he threw Diomedes into the bronze manger, where he was eaten by his own horses. This made the horses calmer, and Heracles easily took them back to King Eurystheus, who dedicated the horses to Hera and allowed them to roam freely around Argos. Bucephalus, Alexander the Great's horse, was said to be descended from these mares.
Alternatively, Heracles brought Abderus and some other youths to help him. They took the mares and were chased by Diomedes and his men. Heracles left Abderus in charge of the horses and fought Diomedes. Abderus was eaten. In revenge, Heracles fed Diomedes to his own horses, then founded Abdera next to the boy's tomb.
Diomedes almost killed Aeneas in battle but Aphrodite, Aeneas' mother, saved him. Diomedes wounded Aphrodite and she dropped her son, fleeing to Mt. Olympus. Aeneas was then eneveloped in a cloud by Apollo, who took him to Pergamos, a sacred spot in Troy. Artemis healed Aeneas there.
Later in the war, Diomedes fought with Hector and saw Ares, the war-god, fighting on the Trojans' side. Diomedes called for his soldiers to fall back slowly. Hera, Ares' mother, saw Ares' interference and asked Zeus, Ares' father, for permission to drive Ares away from the battlefield. Hera encouraged Diomedes to attack Ares and he threw his spear at the god. Athena drove the spear into Ares' body and he bellowed in pain and fled to Mt. Olympus, forcing the Trojans to fall back.
Diomedes and Odysseus later stole the Palladium (and King Rhesus' horses) and took it to Argos. Diomedes also killed Merops' two sons.
When the Trojan War was over, Diomedes returned home. His wife, Aegiale had been unfaithful to him and Diomedes left for Italy, where he founded the cities of Brindisium and Arpus Hippium.
At some point, Diomedes restored Oeneus to the throne of Calydon after his brother's (Agrius) sons had overthrown him.