After fleeing their home from war, Princess Celestia and Princess Luna retreated to Equestria where they were worshipped by the citizens for saving them from the God of Chaos, Discord. They now ruled, but Luna thought it ws wrong to abandon their home. Celestia did not want to go back, they now had a duty as mortals.  
Princess Luna grew to hate Equestria and was possessed by Virnah, Spirit of Vengeance, transforming the moon goddess into Nightmare Moon. This form of her is an angry and powerful being hell bent on war with her twin sister. Attacking her without warning, Nightmare Moon nearly bested her sister, causing a solar eclipse making the early equine people think the world was ending. Celestia only triumphed when she harnessed the power of the Elements of Harmony and banished her sister to the moon where she would stay for a thousand years.  
Now heartbroken from losing the last bit of her family, Celestia was forced to rule the young kingdom by herself.