Gre'thor is the name of the underworld, or Hell, in Klingon society. It is the location where the dishonored and the damned would journey to, once they died. (TNG episode: "Devil's Due", DS9 episode: "Sons of Mogh") The gates of Gre'thor were guarded by the Fek'lhr beast (TNG episode: "Devil's Due"), with the insignia of the location being that of the inverted flag of the Klingon Empire.