In the myths concerning the labors of Heracles, the eighth task he must perform is to steal the mares of Diomedes, King of Thrace. Horse theft should be a simple matter for the son of a god, but these are man-eating horses. Unaware that the horses are mad and deadly, he leaves them with his companion, whom the horses kill and devour. As a punishment on Diomedes for raising such monsters, Heracles feeds Diomedes to his own horses. This is the subject matter for Moreau’s painting, with Heracles nonchalantly looking on as the horses take his revenge. Moreau is famous for his symbolist paintings of Biblical and mythological scenes, though none are as bloody as this one.