Terran Empire

The Terran Empire was a repressive Terran dominated government and mirror universe counterpart of the. .

Star Trek: Odyssey (fan fiction)
This Terran Empire is from another parallel universe where they were never conquered by the Klingon-Cardassian Alliance 

Spock's new vision of reform only lasted until his own Captain Kirk returned to the mirror universe. The mirror Kirk, realizing he had been in another universe and the Empire could conquer that one as well, had Spock killed for his ideas and later became Emperor himself. (Star Trek: Odyssey (fan fiction): "Objects in the Rear View Mirror")

The Terran Empire started the war with the Dominion and took control of the Dominion. However, unlike the Federation, the Empire did not send Voyager into the badlands because there was no reason to send them which resulted in Voyager staying in the Alpha Quadrant and no contact with any Delta Quadrant species except the Borg. (Star Trek: Odyssey (fan fiction): "Advent")

This particular version of the Terran Empire would not make anymore contact with their Federation counterparts until 2380 when Counselor Christine Nycz was sent on a mission to the other side aboard USS Alexandria to kidnap Lt. Commander Dustin Zofchak for information he possessed about the transphasic torpedo. They wanted the transphasic torpedo so they could deal with their own Borg problem. (Star Trek: Odyssey (fan fiction): "Objects in the Rear View Mirror")

Now with the transphasic torpedo in their hands, they took the fight to the Borg and eradicated the Collective. They have set up some outposts in the Delta Quadrant to have a foothold in that area of space as well. (Star Trek: Odyssey (fan fiction): "Advent")

During Zofchak's torture interrogation, he coined the phrase "Mirror Universe" to describe the other side; the mirror Captain Allensworth even liked the phrase enough to start using it himself. (Star Trek: Odyssey (fan fiction): "Objects in the Rear View Mirror")

On stardate 57645.8, forces from the Terran Empire made their way into Federation space in the Talos star system. A battle ensued with both versions of the Alexandria going head to head. Due to the existence of the battle, the Mirror War between the Federation and the Terran Empire had officially begun. (Star Trek: Odyssey (fan fiction): "Advent")

The Empire gained its first foothold in Federation space by landing ground troops, led by Admiral Janeway, on the Ba'ku planet. The assault lasted a few weeks before they were thwarted back into their own universe. (Star Trek: Odyssey (fan fiction): "Requiem ") Other planets the empire had occupied were Betazed (Star Trek: Odyssey (fan fiction): "Where Angels Fear to Tread ") and Cestus III. (Star Trek: Odyssey (fan fiction): "Catalyst of Sorrows") An undercover Commander Merriell was able to get the defense codes and transmit them to the Voyager, where they were able to defeat the imperial forces and push them off the planet.

The Empire then came up with the plan to kill Captain Kirk during the Khitomer crisis using the Guardian of Forever but this plan was stopped when Captain Allensworth, Commander Merriell and Lieutenant Commander Zofchak went back as well and prevented the assassination. (Star Trek: Odyssey (fan fiction): "Held in the Hands of Forever ")

The mirror Tuvok traveled to the primary universe and gave the Federation information pertaining to the date when the empire planned to attack Earth. (Star Trek: Odyssey (fan fiction): "Ghosts of Tomorrow")

The empire attacked Earth to find a fleet of Federation starships waiting for them, however the empire still had the advantage. That was, until the USS Alexandria-A showed up with more starships as well as Klingon and Romulan ships. Unknown to the empire, the Federation had sent four fleets to the mirror universe to attack the mirror Earth and take the Emperor and Empress prisoner. The Emperor accepted defeat and officially declared the Mirror War was now over and the Federation had won. (Star Trek: Odyssey (fan fiction): "Where Mortals Are Forbidden")

A Debt of Honor
In this contiuity Spock became the Empire's commander-in-chief as portrayed on DS9, but was overthrown by the Imperial Starfleet Military because of the influence the Halkans carried in his government. David Marcus (mirror) attempted to carry on Spock's vision of reform through an underground resistance, but made a tragic miscalculation.

The Final Days/The Gift
As in A Debt of Honor Spock was overthrown because of the influence the Halkans carried in his government. During a subsequent war with the Alliance, the Empire attempted to secure an Iconian gateway, which backfired when the planet sent up probes that disabled the fleet's software. What the Empire had left wasn't enough to defend it, leading to its defeat.