Princess Luna is the younger sister of Princess Celestia. Originally, the two ruled together, but Luna began to feel that Celestia was overshadowing her. Her anger and resentment caused her to transform into Nightmare Moon, and Celestia was forced to banish her to the moon for a thousand years. After being freed by a cosmic alignment, she returned to seek revenge, but was defeated by six exceptional ponies bearing the Elements of Harmony. In the process, she was separated from Nightmare Moon, and welcomed back by her overjoyed sister. From then on, the two ruled on a more equal footing, although Luna could still feel the old resentment from time to time (albeit never as great as before).