Title: A Match Made in Hell

Characters: Princess Celestia, Princess Luna, King Sombra, Starswirl.

Cover Art: by Drako1997
http://drako1997.deviantart.com/art/A-Match-Made-in-Hell-460339299

Genre: Romance, Adventure, Alternate Universe, Dark.

Premise:
During another raid on one of King Sombra’s strongholds, Celestia's Knights retrieve a strange mirror the the King discovered. While studying it, she sees a vision of a more benevolent version of herself fighting a more tyrannical Sombra.

Fascinated by this, she visits the new world when the portal opens and confronts the King. But instead of fighting, the two actually get along very well and slowly start making plans to bring both this Equestria and the Mirror one under their rule...