In the study of mythology and religion, the underworld (gr: κάτω κόσμος) is a generic term approximately equivalent to the lay term afterlife, referring to any place to which newly dead souls go. In most cultures the term refers to a neutral or dystopic realm of the afterlife, instead of a heavenly one. Sometimes the underworld is identified as "Hell" because Hell was thought to be under the Earth.
(Note: this includes guardian-type creatures, ghosts, and spirits such as demons, veli, and Cerberus)