Hades is the Greek god of the underworld. He obtained the underworld when he and his siblings battled the Titans for earth. When he won, the siblings each took part of earth to rule, and he got the underworld.
Hades is in charge of deciding where people go after they die and making sure everyone that dies stays in the place that Hades put them in. His dog Cerberus also guards the gates of the underworld to prevent the living from entering and the dead from leaving.
Where the people that behaved the worst in their lives went and had committed crimes against the gods. Hades specially picked each eternal punishment for each of them.