Harkoum is a Cardassian planet in the Geilod system, particularly distant from core worlds such as Cardassia itself. The planet orbits two stars, making the planet incredibly warm during the day. Due to the intense heat, Harkoum's surface is covered by deserts. Despite this, Harkoum was known for its several industries.
In the 23rd century, Harkoum was one of the Cardassian Union's biggest producers of raw minerals and refined alloys such as duranium. The planet was uninhabited, making it useful for the Cardassians to use it for large scale industrial operations, including strip mining and weapons manufacturing. By the end of the 23rd century, Harkoum's refining and manufacturing days were over, the planet having been bled of resources. (DS9 novel: Warpath)
The planet soon became home to several prison facilities, such as the Obsidian Order's prison facility Grennokar. Among the unfortunate people sent there included criminals, dissidents and aliens. Some prisoners were executed, although others were used for hard labor. There were also Cardassian scientists based there who subjected prisoners to biogenic weapons, new surgical techniques and as test subjects for refining torture techniques. (DS9 novel: Warpath)
In 2377, Taran'atar hijacked the USS Euphrates and later the IKS noH'pach on his journey to Harkoum to meet with Ghemor. Elias Vaughn and the crew of the USS Defiant pursued the Jem'Hadar, but were unable to stop him from transporting to the mirror universe. However, Vaughan and Prynn Tenmei escaped the destruction of Grennokar and encountered the Iliana Ghemor of the alternate universe. (DS9 novel: Warpath)