In the fictional Star Trek universe, Gre'Thor is the Klingon afterlife where the dishonored go when they die. It is the Klingon equivalent of hell and is guarded by a fearsome demon, Fek'lhr (pronounced "Fek-Lar"). Those unfortunate Klingons who find themselves in Gre'Thor are faced with eternal torture by Fek'lhr and his demons; however, Klingon legend allows for souls to be saved from Gre'Thor, usually by heroic sacrifices performed by friends and family (Kahless, for example, once willingly journeyed to Gre'Thor to save his brother, Morath, and send him to Sto-Vo-Kor). The Enterprise boldly going where no man had gone before. ... Klingons (tlhIngan in the Klingon language), are a race of humanoids in the fictional Star Trek universe. ... Klingons (tlhIngan in the Klingon language), are a race of humanoids in the fictional Star Trek universe. ... Medieval illustration of the Mouth of Hell Hell (according to many religious beliefs about the afterlife) is a place of torment and pain. ... Kahless, (qeylIS in the Klingon language), is a legendary Klingon portrayed in the Star Trek series Star Trek: The Next Generation by Kevin Conway. ... In the fictional Star Trek universe, Sto-Vo-Kor is the state of existence that Klingons believe that honorable members of their society will go to in the afterlife. ...

If a Klingon dies and is fated to journey to Gre'Thor, they will find themselves on the Barge of the Dead, which travels the 'river of blood' on its way to Gre'Thor. Klingon souls on the Barge are tempted by siren-like voices, masquerading as friends and family, who try to lure them off the edge and into the river. (As the Barge's pilot, Kortar, wryly points out, "There are things here much worse than death...") Klingons (tlhIngan in the Klingon language), are a race of humanoids in the fictional Star Trek universe. ...

Under normal circumstances it is difficult for the souls of dead Klingons to leave Gre'Thor, such as in the expression "I will surrender when spirits escape from Gre'Thor!" but the legend of Kahless does allow for such things.