This is just an experiment in writing I wanted to try. This is infact based on a picture I dreamed about drawing. This is based before Equestria Girls, and before season 3, so, the mirror is still in Canterlot.
Also, this is just my headcanon, but the mirror can connect to any magical world, as long as the pony who uses it focuses on it.
Luna walked the halls of the castle at night, her hair flowing with her as she walked around. She knew the signs. A new nightmare would awaken again. Another pony would scream in horror and lose more of themself in it. She walked to the mirror and put her hoof to it. She had been battling the nightmares for centuries. However, she had fallen to the nightmares soon after her sister and herself had defeated Discord. Because of that, she became the ultimate nightmare..
She had become Nightmare Moon.
The pure embodiment of darkness and death. Her power was so strong, it affected multiple dimensions, spreading nightmare all over.
Luna closed her eyes and her horn began glowing. Soon, the mirror began to glow, revealing people who had been affected by the nightmare. Her eyes saddened, seeing most of them had actually been fairly young children.
There was a boy with blue hair, pale skin and blue eyes. He had fallen into a coma due to the nightmare and in another world, became an embodiment of nightmare as well.. "Duskmon.."
A girl with long red hair with wings was next. She wore an outfit of blue and was a princess, much like Luna. While in college, she encountered another embodiment of nightmare. "Darkar.." He had corrupted her and turned her against her friends, almost killing the man she loved.
Next was a merman. He had long lilac hair and had fallen to nightmare as well. "Tritanius.." He had been turned into a horrible monster and had desired to take over the entire dimension of the red haired girl.
Finally, the last was a young man, no older than 'Duskmon' was. His brother had died at a young age and ever since then, the grief of it all had caused the nightmare to take over. "Digimon Kaiser.."
She turned away and closed her eyes, "They are the only ones who actually know the pain of facing the nightmare.. I did it alone for over 1000 years.." She turned back to the mirror and her horn glowed, putting a bag on her back, "But, they will not face to!" She jumped right into the mirror, just as the moon rose to it's peak.