Title: Shattered Heart, Fractured Love

Main characters: The Mane Six, Spike, Princess Celestia, Princess Luna, King Sombra, Discord, Queen Chrysalis, Trixie.

Genre: Alternate Universe, Romance, Dark, Adventure.

Premise:
After draining Evil Celestia and Luna of their dark ways and absorbing it into himself, King Sombra retreats to his castle and cuts off all contact to the outside world. While Ra Celestia & Artemis Luna attempt to bring the balance to their world, the now-villainous King manages to contact the original Celestia through the remaining shard of the broken mirror, slowly manipulating and deceiving her with sweet-sounding lies of wanting to be together with her again, eventually corrupting her with his magic.

Soon Sombra has Celestia reconstruct the mirror, however the damage done to it was too great. As a result she ends up drawing the power from her own sister, when she tries to resist Celestia forcibly takes her power, rebuilding the mirror and becoming just as evil as her counterpart…

The Mirror Sombra crosses over the Prime Equestria and immediately begins his hostile takeover with Celestia by his side, intending to bring both worlds under his control. Will the Element’s be able to combat this new threat, especially when that threat is the Princess herself? Or maybe they will have to gain assistance from the Mirrorverse as well...

I might try something with that as I found a possible cover image.