Terran Empire

From MU

The Terran Empire was a fixture of the Mirror Universe. While there is no canon date for establishment, in the canonical timeline, an encounter with the normal universe's Enterprise caused Spock to ascend to the position of Emperor, so he could dismantle the Empire from within. Eventually this would lead to a powerful Klingon-Cardassian Alliance ruling most of the Alpha quadrant.

In an alternative timeline, Spock did not dismantle the Empire, instead using diplomacy and military might to conquer most of Alpha and Beta quadrants. The Empire has recently found MU, and agents are coming through time and across the dimensional boundaries to investigate and attack.