A thousand years ago, Princess Celestia banished her sister to the moon, that much we know. What wasn’t known, and what Celestia kept secret was that before her banishment, Luna had given birth to a foal; Twilight Andromeda Sparkle.
King Sombra, the tyrant of Crystal Empire is back, only to be imprisoned by princesses and put through reformation with Twilight Sparkle as his teacher. Can somepony with heart as black as night be reformed? Twilight believes so.