Princess Celestia is generally portrayed in My Little Pony Friendship is Magic as one of the good guys (or gals or ponies?). However in the MLP collector's card game, she has a card where she is portrayed as an evil version of herself: Nightmare Star.

What if both Celestia AND her sister Princess Luna (reverting to Nightmare Moon) reverted to evil?

Equestria is plunged into civil war as Nightmare Star and Moon, possibly with some loyal royal guards, fight against the Mane Six as led by Twilight Sparkle. Princess Cadance very likely would ally herself and the Crystal Empire with Twilight.

Discord would be a wild card: would he ally with the Mane Six or the evil versions of Celestia and Luna?

Queen Chrysalis and her Changelings would be yet another wild card: they probably would fight both Nightmare Star AND Twilight Sparkle.