Hercules was ordered to fetch the red cattle of Geryon, son of Chrysaor by Callirhoe, daughter of Ocean.
Geryon was a monster with three bodies and three heads.
His cattle were guarded by Orthus (Orthrus) a two-headed dog and a herdsman,
Eurytion. (It was on this trip that Hercules set up the Pillars of Hercules at the border between Europe and Libya.) Helios gave him a golden goblet to use as boat to cross the ocean.
When he reached Erythia, the dog Orthus rushed at him. Hercules clubbed the hound to death and then also the herdsman and Geryon. Hercules rounded up the cattle and put them into the golden goblet and sailed back.
In Liguria, sons of Poseidon tried to rob him of the prize, but he killed them. One of the bulls escaped and crossed over to Sicily where Eryx, another son of Poseidon, saw the bull and bred it with his own cattle. Hercules asked Hades to watch the rest of the herd while he rescued the errant bull. Eryx wouldn't return the animal without a wrestling match. Hercules agreed, easily beat him, killed him, and took the bull. Hades returned the rest of the herd and Hercules returned to the Ionian Sea where Hera afflicted the herd with a gadfly. The cattle ran away. Hercules was only able to round up some of them, which he presented to Eurystheus, who, in turn, sacrificed them to Hera.