King Diomedes had a herd of untamed horses that could run as swiftly as the wind. Herakles' eighth labour was to sail to Thrace, where the king lived, and capture four of the horses and bring them back to Eurystheus.

What Eurystheus had not told Herakles was that the horses were fed on human flesh. He had told Diomedes that he could feed Herakles to the horses. Being clever, Herakles suspected that the two kings were setting a trap for him so he decided to trap Diomedes first.