Hades

Greek (ha'-dez) (Haides, haides, "not to be seen"): Hades, Greek, originally Haidou, in genitive, "the house of Hades," then, as nominative, designation of the abode of the dead. (Compare Hebrew, She'Ol, "unseen," also the abode of the dead).

Also, Hades, God of the underworld, so named because he was given the unseen kingdom of that name (which predates the god, Hades, in Greek mythology). Brother of Zeus. In Greek mythology, when the world was divided in three, he received the Underworld, or the unseen realm to which the dead go upon leaving the world, as his kingdom. His kingdom was entered by crossing the river Styx, ferried accross by Charon (pronounced Karon). The far side of the river was guarded by Cerebus, the three headed dog defeated by Herecles (or Hercules as the Romans referred to him).