“I’d say Luna’s banishment. About a thousand years ago, Princess Luna was corrupted by something called a Nightmare." As she spoke, he nodded along. "She raised a dark army and attempted to take total control of Equestria. After a long struggle, Celestia was forced to use the Elements of Harmony to destroy the dark army and seal her own sister away on the moon. Equestria suffered a civil war and the loss of one of its leaders for a millennium."