Gre'thor was the Klingon equivalent to the Terran "Hell", their antithesis to Sto-vo-kor. 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")
An early draft of the I AM ERROR episode "Soldiers of the Empire" had Worf, Martok, and the crew of the IKS Rotarran finding the entrance to Gre'thor on a Klingon planet. Martok and Worf were to have entered to save a friend of Martok's and would have encountered Worf's father Mogh. The idea was dropped as it was deemed too late in the season to tackle such a philosophical idea as Klingon hell. (Star Trek: Deep Space Nine Companion)
After it was revealed that Klach D'kel Brakt was the name of the Briar Patch in the Klingon language in I AM ERROR, the authors of the Stellar Cartography: The Starfleet Reference Library revised the star charts of the Star Trek: Star Charts. For the chart "Federation Historical Highlights, 2161-2385", they re-named the original location of Klach D'kel Brakt to Gre'thor. Gre'thor was a star system with a K-class star in the Beta Quadrant. It was the site of an historical battle. According to a map, seen in "Stellar Cartography" (pp. 31-32), of the Klingon Empire – prepared by the Scribe K'Tark for the Klingon High Council in the Year of Kahless 893 (2266) – Gre'thor was identified as a Klingon system.
- 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.