The Mirror Realm is a dimension that runs parallel to the real world.
The Mirror Realm is a desolate and decaying place of ruin, with barely any life. It had been ravaged by a large conflict. In the Mirror Realm, there are three entities other than the scratches of life left. There are ponies, like the ones in the real world, that can be friendly or hostile and are very few in number, natural creatures created from the remains of the conflict, and robotic minions (originally built to protect the ponies, but defected) that walk the land and attack anything that they see moving. Most of the world is just a barren gray dusty rock, with a few volcanoes and mountain ranges, however, there are a few select places where natural life has survived, but even that is slowly dying.
The Mirror Realm, hence it's name, can be accessed naturally every 100 years when portals to the Mirror Realm open randomly in mirrors all around the real world. These mirrors can be absolutely any size, from pocket mirrors to large room sized mirrors. There are also mirrors in the Mirror Realm that open and have the same properties, however, these are indestructible by normal means, and they are mostly located outside in the middle of nowhere. They can also be willingly opened from the Mirror Realm is someone needs to get back. Portals to the Mirror World can be created from the real world with the use of powerful magic, only known to a select few ponies who have mostly passed by now, one of which was Starswirl the Bearded.