The three god brothers - Zeus, Poseidon, and Hades - cast lots to divide the world among them, and Hades became the King of the Dead in the underworld.
Research Tip: The Theoi.com website has a section devoted to the Underworld Gods and Goddesses, and Wikipedia has articles you will want to look at: The Greek Underworld, Hades, Persephone, Cerberus, etc.
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