Hell

Hell was the underworld in a number of human religions, a place where those who did not obey religious and moral codes were sent after death. Hell was believed to have been ruled by the devil, a fallen angel.

One of the illusions that an uncooperative Captain Christopher Pike was subjected to was one of hell. Years later, the Talosians realized that they had made a horrible mistake, given that Pike's mother had died in a fire when he was a child. (TOS episode The Cage and novel Burning Dreams).

A common human curse involved telling another person to "go to hell." A variation of this was to tell a person to "go to the devil." Captain Kirk once told Kang this when Kang demanded Kirk's surrender. Kang responded that the Klingons did not have one, but understood the habits of the human's devil. (TOS episode Day of the Dove).

In Klingon culture, the dishonored of their society were sent to Gre'thor following death. The Klingon religion did not a devil like the one found in human culture, but did have a guardian known as Fek'lhr. (TOS episode Day of the Dove and TNG episode Devil's Due)