Gre'thor was the Klingon equivalent to the Terran "Hell". It was where the souls of the dishonored went when they died after being ferried there on the Barge of the Dead in the waters inhabited by Kos'karii. The insignia of Gre'thor, and dishonored Klingons, was an inverted emblem of the Empire.
In 2372, after he and his family had been stripped of their honor, Kurn considered that if he were cast down into Gre'thor, he'd have had the consolation of being with other Klingons, dishonored or not. (DS9: "Sons of Mogh")
In 2376, B'Elanna Torres experienced a near-death experience about her mother Miral almost being sent to Gre'thor because of her daughter's dishonor. However, Torres traded her life for that of her mother, and she herself was sent to Gre'thor, which to her was interpreted as USS Voyager and its crew. (VOY: "Barge of the Dead")
- Sto-vo-kor, the Klingon afterlife for the honored dead
- In the PC game Star Trek Online, KDF players who reach the rank of captain can enter Gre'thor to fight the Fek'lhri.
- Occasionally the acknowledgment "Gre'thor will be paved with their ashes" is said when ordering certain Klingon ships to attack the enemy in the game Star Trek: Armada.