Hades

Hades was the infamous god of the dead, the ruler of the Underworld. In classical Greek, his name is always that of a person, never a place, although later, by extension, Hades came to be the name for the Underworld itself. (This happened through an elliptical use of the name in the genitive, meaning 'of Hades', with the missing but understood first word being 'house'.)

The original form of the name was Aides, this meaning 'the unseen', from a-, 'not', and eido, 'to see' (this latter verb being ultimately connected with Latin video). This 'invisibility' was by implied contrast with the sun, Helios, who was clearly visible.

In spite of this generally accepted derivation, however, there have been other suggestions regarding the origin of the name, one of them being a development from aianes, 'everlasting', 'wearisome', 'horrible'. This certainly conjures up an unpleasant endless torment in a version of Hell.