The Crystal Empire

The Crystal Empire has returned after a thousand years, and Dark King Sombre is chomping at the bit to return to the Crystal Throne. But wasn't Luna bashished to the moon for... a thousand years?

What exactly happened a thousand years ago, and does it have anything to do with Nightmare Moon's appearance? Who was Sombre exactly? Why were Luna and Celestia so concerned with Twilight Sparkle's test? We have a theory that ties them together.