Ko'th was a Klingon belief in the afterlife that can be compared to limbo. It was a state of non-being inbetween Gre'thor and Sto-vo-kor where the damned would remain.

To escape from Ko'th, one must commit an act of vengeance. A rightful act will lead the deceased to Sto-vo-kor while a wrongful act of vengeance will lead the damned to Gre'thor. It was said that even Gre'thor was more preferable to a life in eternity in Ko'th.

(DS9 novel: Millennium Inferno)