 He is known for his man-eating horses, which Heracles stole in order to complete the eighth of his Twelve Labours, slaying Diomedes in the process.Heracles encounters King Diomedes through performing his eighth labour.Eurystheus, King of Tiryns and Heracles' cousin, had sent Heracles to capture the Mares of Diomedes after he had completed his seventh labour, capturing the Cretan Bull.