In the mirror universe, Qo'noS is the homeworld of the Klingon people. (ST novel: Dark Passions, Book One)
According to Through a Glass, Darkly, Qo'nos was conquered by the Terran Empire in 2218 after a campaign beginning in the late 22nd century.
It was in the late 22nd century that the ISS Ranger warped into the Qo'nos system and arrogantly demanded the surrender of the Klingons. The Klingons agreed, and only asked to be allowed to surrender through their own sacred ceremony. Instead, they seized the Ranger and stole its technology, using their slave worlds to fashion copies of the ship.
Emperor Louvin offered the Klingons a favored role within the empire; however, the legendary Klingon stubborness and obstinacy prevented them from accepting it and forced the empire to continue the assault. In the end, the Klingons were unable to hold off the Terrans and their allies; nine out of ten Klingon ships were destroyed, and their core colonies smashed. Eventually Imperial Starfleet was pushed its way to Qo'nos where General Order 24 was carried out, destroying all life on the planet and turning the Klingons into a race of refugees. With their homeworld conquered, the empire left the few remaining Klingons alone (the Klingons were scattered to the most distant and remote surviving outposts). Now in possession of Qo'nos, the Terran Empire undertook to re-populate the planet using Human colonists.
Over time, the surviving Klingons focused their efforts on not only seeking revenge, but increasing their limited numbers. Steadily, they began to make raids into the empire's territory, stealing ships and supplies. With the formation of an alliance with the Cardassians, the Klingons were ready to take back their lost homeworld.
In 2293, Qo'nos's moon, Praxis, is blown up in an act of terrorism by the Klingons using Khak'ma bombs. During the resulting chaos on the planet's surface below, Klingon ships massacre the remaining Human colonists using strafe tactics and reclaim their homeworld. Upon reoccupying Qo'nos, the Klingons begin the process of rebuilding many large cities, including their capital, the First City. The entire planet is remodeled into a dark-majestic fortress world.
By the late-24th century, the mere mention of Qo'nos is enough to strike fear in the many subject-worlds of the Klingon-Cardassian Alliance. Gowron, the Intendant of Qo'nos and its system, now rules over the Klingon people. Moreover, unlike his predocessor, Intendant Worf, Gowron rules Qo'nos with a combination of cleverness and subtlety. His enemies never know when he will challenge them to simple combat, use blackmail, or send an assassin to thrust a kut'luch between their ribs. It is also rumored on Qo'nos that Gowron further relies upon the Duras sisters, Luras and B'Etor, as Enforcers of his will. The appearance of the Duras fleet is enough to sway any planetary system to the Intendant's will.
