Reflections universe

The Mirror Universe was one of infinite universe throughout the entirety of the Multiverse. It was seen as a dark reflection of the "Main" Universe of sorts, though not necessarily an evil one.

History
Over 1,000 years before the Fateful day, both $𝜇$ and $𝜇$ fell to Nightmare, becoming evil, corrupted versions of themselves. At some point, $𝜇$ rose to the rank of king by the will of his people, and Luna turned from evil to serve as his advisor. During his rule, Sombra fell in love with the dimension-hopping and formed a friendship with her companion, Starswirl the Bearded. As their travels between the two worlds began to affect both, Starswirl attempted to limit what they did, but Celestia's love led her to travel alone against his wishes, drawing the two worlds closer together.

Eventually, when was freed from her Corrupted state as Nightmare Moon, Mirror-Luna returned to evil and rejoined her sister's efforts to conquer their $𝜇$. After laying $𝜇$ and other cities to waste, the pair grew tired of terrorizing Sombra's citizens and sought a new world to conquer. To that end, Mirror-Celestia attacked her counterpart when the latter came to visit Sombra, leaving her injured and weak. Mirror-Celestia then offered Sombra a deal: allow her and Mirror-Luna to travel to Celestia's world, and they would leave the Mirror Universe in peace and allow Celestia and Sombra to be together.

Despite his love for Celestia, Sombra was unwilling to let the sisters loose on an unsuspecting Equestria, and so sent Celestia back to her own world. She then sent the Mane Six and back through the Magic Mirror she used with the Elements of Harmony, which did not exist as physical items in the Mirror Universe. The Mane Six briefly attempted to attack Mirror-Celestia and Luna, only to learn that any harm done to them would also be inflicted on their own world's Celestia and Luna. After discussing the matter, the Mane Six and Sombra hit upon a plan to encase the Mirror-Princesses in crystal, unaware that Mirror-Luna had eavesdropped on their plans.

The heroes soon confronted their adversaries at the $𝜇$, where Mirror-Celestia again offered Sombra the chance to accept their deal. When he refused, she decided to lure Celestia back to the Mirror Universe by attacking Mirror-Luna, which in turn caused harm to the Luna of the other world. It also broke Celestia's mirror, and the two worlds began to merge together; meanwhile, an angry Celestia attacked her counterpart. When the Mane Six tried to enact their plan, Celestia began to be covered in crystal as well, and Sombra decided there was only one means of saving both worlds.

Taking the Elements from the Mane Six, Sombra used them himself to draw the Nightmare energy out of Mirror-Celestia and Luna, causing them to revert to a natural good state. Unfortunately, Sombra was forced to draw the corruption into himself, thus making him a double for his own counterpart from Celestia's world. With the worlds separating and no longer connected by a mirror, Mirror-Celestia and Luna presumably took Sombra's place as noble monarchs. Sombra, on the other hand, departed, his future unclear in the wake of his sacrifice.