Diomedes's father, Tydeus, was a member of the Seven Against Thebes. The Seven's war ended in failure, and all of its member's were killed. At their funerals, Diomedes and the other sons of the Seven swore that one day they would vanquish Thebes and have revenge for their fathers' deaths. Ten years later they fulfilled their oaths, and with only a small force, the Sons of the Seven defeated the Theban army and sacked the city.
Diomedes was on the suitors of Helen of Sparta, and thus was bound by oath to join the Achaean forces in the Trojan War. Although he was the youngest commander there, he had the most experience, having fought more battles than any of the others. He and Odysseus went on several covert missions against the enemy, sabotaging them and even sneaking into Troy itself to steal an artifact from Troy's temple.
Diomedes's greatest feat of all was his wounding of two gods. As Diomedes attacked the Trojan Aeneas, Aeneas's mother Aphrodite descended to protect him. With some aid from Athena, Diomedes instead attacked Aphrodite, and Ares as well when he came to help. By a mortal's hand, two gods had been driven back. Diomedes is one of only two mortals to achieve such a feat.
At the end of the war, Diomedes returned to a home where he was not welcome. His wife had fallen for another man, the work of a vengeful Aphrodite. He left Argos, and sailed to Italy, where he founded several cities. At the end of his life, Athena would grant him godhood.
Battle vs. Sigurd (by Laquearius)
Sigurd had some significant advantages in this battle, namely his armor-piercing sword, healing magic, and mind-affecting helmet. However, the voters agreed that Diomedes’s physical abilities, experience, equipment, and the huge power boost from Athena’s blessing gave him enough of an edge to claim victory over the more gimmicky Norse hero.