Mirror-Universe

Eris' dimension is an alternate reality version of the world that is home to Equestria, with Princess Eris-its former ruler-being one of its more notable inhabitants. She served as its lord of chaos, whereas her cousin fills that role in Equestria's world

History
Apart from Eris serving as the world's counterpart to Discord, this version of Equestria seems to have had a similar history-at least in the beginning-to Discord's version. After Equestria was established as a nation, this world's Princess Celestia and Princess Luna were chosen to be its rulers, and mentored by Starswirl the Bearded. Unfortunately, as in the main dimension, Stygian became estranged from the Pillars of Equestria and became the Eris' dimension. Worse yet, when the Pillars of this world attempted to banish him to Limbo as the Pillars of the main universe did, Eris intervened and stole Starswirl's journal, thus preventing them from completing the spell.

Knowing that the Pony of Shadows intended to corrupt Celestia and Luna, Starswirl sacrificed himself to hold off the villain while the other Pillars escaped with them. The other Pillars all subsequently perished, presumably at the Pony of Shadows' hooves, but Celestia and Luna managed to elude their pursuer. Determined to bring his Destroyers into being, the Pony of Shadows began searching other universes for them. He eventually located those of the main universe, abducting by herself and later both her and. However, the Starswirl the Bearded of that reality, accompanied by his fellow Pillars of Equestria and Stygian, succeeded in rescuing the pair.

Undaunted in his efforts, the Pony of Shadows sought Eris' aid in his plans; when she refused, he imprisoned her and used her Chaos Magic to increase his own power. He eventually succeeded in capturing two of the princesses, and corrupted them into Daybreaker and Nightmare Moon. However, Nightmare Moon eventually rebelled against him, and with the aid of Daybreaker he destroyed her. Unfortunately, the Pony of Shadows turned to Eris for aid in preventing Daybreaker from following her sister's example, only for Eris to gain control of the fallen Alicorn and the two of them to destroy their "master".

Now freed, Eris turned to subjugating a world already corrupted by the Pony of Shadows' influence, turning it into the ideal center of her own destructive brand of chaos. This included establishing her castle, the Chaos Roadhouse, as a casino where villainous-or formerly villainous-patrons from various realities could visit. With her royal guards enforcing what order was to be found and Daybreaker as her chief of security, Eris was able to enjoy her new position. However, she was always out to find new means of amusing herself, and eventually turned her eyes to the same universe that had thwarted the Pony of Shadows on two separate occasions.

Infiltrating the dreams of Stygian, Eris posed as the Pony of Shadows she had defeated in order to lure that world's Princess Luna to her casino. With Daybreaker as bait in her trap, she soon captured Luna, and used the Staff of Sacanas to drain away much of her magic. She intended to auction it off to the highest bidder, but her triumph was interrupted by Stygian, who had accompanied Luna, helping the weakened princess to escape. The pair returned to their own world, but returned some time later with help: the Nightmare Knights.