After killing his family in an insane rage, Herakles (also known as Hercules) received orders from the oracle at Delphi to perform 12 labors to purge his sin. These were assigned by Eurystheus, king of Mycenae.
The tenth labor involved traveling west to Erythia (some where beyond present day Spain) to kill Geryon, a giant with three bodies and and three heads. The giant lived peacefully with a herdsman, and his two headed dog, Orthrus, tending large herds of cattle.
Eurystheus wanted the cattle, but without the giant. Herakles set out to find the land. On the way, he placed two rocks at the entrance of the Atlantic (now called Gibraltar). Wandering through the desert, he grew angry at the sun, shooting arrows at Apollo. Apollo eventually sent a golden boat to transport him to Erythia.
The dog and herdsman were easy to kill, but the giant needed to be killed three times, as it were. Herakles then traveled home, driving the huge herds before him. Traveling through Italy, he passed near the cave of Cacus, son of Hephaestus. Cacus took the eight best animals, dragging them backwards by their tails so as not to leave obvious tracks, while Herakles slept.
Indeed, the only way our hero found the missing animals was by following the sounds of their lowing as the main herd passed by. As Herakles approached, Cacus emerged from his cave. Did I say he was another giant, one who could breath fire, and who had the bad habit of maintaining a trophy room filled with his victims' heads? A single blow of Herakles' club, and yet another monster lay dead.
Click here for the eleventh labor.