The Crystal Empire disappeared long ago during Princess Celestia and Princess Luna's quest to cleanse the land of Equestria from all forms of evil. King Sombra was a great threat to their entire world and when his defeat came about, at the cost of the Crystal Empire vanishing from existence, it brought nothing but a whole lot of good to the world.

What they didn't realise however, was that Sombra had survived their confrontation and was now plotting his revenge against them both in the one way he knew how.

Twilight Sparkle thought she was the luckiest filly in all of Equestria when Princess Celestia asked her to become her personal student, but she had no idea she'd be a pawn in a game nopony is even aware exists.

Source for art: https://www.deviantart.com/dipfanken/art/Domesticated-626423587