Her sister was not strong enough to overcome Nightmare Moon on her own. She had only one choice. To unleash the most powerful magic known to ponykind. The legendary Elements of Harmony.

Though it grieved her to use such a mighty power against her sister, the princess of the sun knew that if left underneath the rule of Nightmare Moon, her beloved little ponies could not survive. She had no choice. With the Elements beside her, she sealed her sister away... trapped within the very moon she had once commanded. With her sister gone, she had to take control of the moon as well, and so she became ruler of both night and day.

But even as Nightmare Moon was sealed, a prophecy was made, though who it was who made it is a mystery lost to time. It said that a thousand years in the future, the stars would aid in her escape... and if not stopped again, she would bring about nighttime eternal.