Gre'thor is the underworld of Klingon belief, guarded by the monstrous Fek'lhr, to which the souls of the dishonoured are ferried on the Barge of the Dead.
Gre'thor is mentioned in several series as the opposite of Sto-vo-kor, the hall of the honoured dead, and Klingons use the name much as humans speak of hell.
It is depicted in the Voyager episode Barge of the Dead. After a shuttle accident, B'Elanna Torres finds herself on a vessel crossing a sea of blood, piloted by Kortar, the first Klingon, who is condemned to carry souls there for having destroyed the gods. She learns that her mother Miral is bound for Gre'thor because of her daughter's rejection of Klingon ways.
Torres has the Doctor return her to the brink of death so that she can take her mother's place. Her Gre'thor takes the form of Voyager itself, and she must lay down her anger to leave it.