A legendary Greek hero, son of Tydeus and Deipyle (Tydides). He was the favorite of Athena and was under her protection. Diomedes participated in the expedition of the Epigone ("the After-born") against Thebes. Later he succeeded his grandfather Adrastus as the king of Argos and joined the Greeks against Troy. On the Greek side, the two greatest champions were Ajax and Diomedes. They fought gloriously and many a Trojan fell before their weapons. Diomedes nearly slew Prince Aeneas. Aeneas was of royal blood, and the goddess Aphrodite (his mother) hastened to the battlefield to save him. She lifted him in her arms, but Diomedes leaped towards her and wounded her hand. Crying out she let Aeneas fall, and weeping for pain she made her way to the Olympus. Although Aphrodite failed to save her son, Aeneas did not die that day. Apollo enveloped him in a cloud and carried him to sacred Pergamos, the holy place of Troy, where Artemis healed him of his wound.
The battle went on, and Diomedes, wreaking havoc in the Trojan ranks, came face to face with Hector. There to his dismay, he saw the war-god Ares too, fighting for Hector. At the sight of the bloodstained murderous god, Diomedes cried to the Greeks to fall back, slowly, and with their faces toward the Trojans. When Hera saw what was happening, she became very angry and urged her horses to Olympus. She asked Zeus if she might drive Ares from the battlefield, and Zeus, who loved him no more than she did although he was their son, willingly gave her leave. She hastened down to stand besides Diomedes and urged him to smite the terrible god and have no fear. At her words, joy filled his heart and Diomedes rushed at Ares and hurled his spear at him. Athena drove it home, and it entered Ares' body. The god bellowed as loud as ten thousand cry in battle, and at the awful sound trembling seized the whole host, Greeks and Trojans alike. Ares fled from the battlefield and with the war-god gone, the Trojans were forced to fall back.
Diomedes also accompanied Odysseus in the nocturnal raid on Troy, to steal the Palladium; the ancient figure of the goddess and venerated by the Trojans more than anything else. They managed to secretly carry it away, and Diomedes took it with him to Argos (but in other stories, it was Aeneas who took the Palladium with him).
When he returned home he discovered that his wife Aegiale had been unfaithful to him. He left Argos and arrived after some journeys in southern Italy where he supposedly founded several cities, among which Brindisium (Brindisi) and Arpus Hippium (Arpi).