In Klingon mythology
, the place where the dishonoured go to die.
In other words, a Klingon's hell. The mythic Klingon figure, Fek'lhr, was said to guard Gre'thor.
In one of the Voyager episodes, B'Elanna had gone on the Barge of the Dead which would eventually bring her to Gre'thor. This was a result of her dishonour towards the Klingon culture that she had always avoided.
See also: Fek'lhr