The alliance of Klingons and Cardassians that overthrew the Terran Empire in the Mirror Universe and reduced humans to slavery.
In the Mirror Universe, the reforms introduced by the mirror Spock after his encounter with Captain Kirk left the Terran Empire unable to defend itself. The Klingons and Cardassians formed an alliance that conquered the Empire, and by the 24th century humans were a subjugated, enslaved people. Bajor became a significant power within the Alliance.
When Kira Nerys and Julian Bashir crossed into this universe in 2370, they found Terok Nor run as an ore processing station by the Intendant, Kira's ruthless counterpart, with Terran slaves forced into labor. Bashir was put to work in the ore processing center, while Kira learned the history of the Alliance from the Intendant herself.
The Alliance remained the backdrop of several later Deep Space Nine visits to the Mirror Universe, including a Terran uprising that eventually seized Terok Nor and a war that ended with the Alliance under pressure from the rebellion. Its regent, a mirror version of Worf, appeared in the final mirror episode.