Celestia embodies the sun, she is its guardian. Luna is the goddess of dreams, guiding minds in the realms of fantasy. Cadance protects the heart, and shines her love across her kingdom. Twilight controls and regulates the flow of magic around the world, and together they are the four princess of Equestria.

...But what of the fifth?

I am the fifth, and the last. I am Nightmare Moon. I am not a princess yet though, and this story tells how I became what I am today, Princess Moon, the bearer of the crown of shadows. It was not an easy task mind you, gaining my crown, and many obstacles lay in my way, but just as the world needs Luna to guard their dreams, so too am I needed.

I am the princess of nightmares, and they need a leader to guide them.