In Greek mythology, Diomedes ("god-like cunning") was the son of Tydeus and Deipyle and a favored hero of Athena. He was one of the Epigoni and later became King of Argos, succeeding his grandfather, Adrastus.
In the Iliad, Diomedes is one of the most attractive figures among the Greek generals. Along with Sthenelus, he leads the Argive armies. He has a horse named Lampos. One of his companions, a brother-in-arms, is named Euryalus.
Book V of the Iliad is centered on the battlefield valor of Diomedes. He duels with Aeneas and nearly kills him, but Aphrodite, Aeneas' mother, comes to his aid and tries to cover him up. Diomedes wounds Aphrodite and she drops her son, fleeing to Mount Olympus. Aeneas is then enveloped in a cloud by Apollo, who takes him to Pergamos, a sacred spot in Troy. Artemis heals Aeneas there.
Later in the same melee, Diomedes fights with Hector and sees Ares, the war-god, fighting on the Trojans' side. Diomedes calls for his soldiers to fall back slowly. Hera, Ares' mother, sees Ares' interference and asks Zeus, Ares' father, for permission to drive Ares away from the battlefield. Hera encourages Diomedes to attack Ares and he threw his spear at the god. Athena drives the spear into Ares' belly: bellowing in pain, the wounded god ascends to Olympus in a column of smoke, forcing the Trojans to fall back.
Subsequently, in a night raid on the Trojan camp, Diomedes and Odysseus steal King Rhesus's team of fine horses. This demonstrates the two kings' courage and guile, but more importantly fulfills one of the prophecies required for the fall of Troy: that Troy will not fall while the horses of Rhesus feed upon its plains.
Like the other major Greek characters, Diomedes is alive and well as the Iliad comes to a close. In the Odyssey the reader is told that when the Trojan War was over, Diomedes returned home. Other stories of his exploits come from later classical sources.