Hades ruled the underworld as the son of Cronus and Rhea and brother of Zeus and Poseidon. As lord of the underworld, Hades was known by the name Pluto and guarded the realm with the three-headed dog Cerberus. The River Styx separated the earth from the underworld, where the ferryman Charon transported souls of the dead, including Achilles whose only weakness was his heel. Hades' wife was Persephone, the daughter of Zeus and Demeter who became Queen of the Underworld.