Princess Luna is a beautiful princess of the night. She is one with the moon. She is kind and pure. Nightmare Moon is an evil, maniacal demon who takes over her heart. The heart, while extremely important chooses to let ponies in... even the ones that do the most damage. Nightmare Moon loathes the light and anypony who basks in it. What do you do when you don't like something? Ignore it or complain? Evil demons kill everything in their way. Unfortunately, the main thing getting in her way is a series of brave ponies who don't know what they're getting into. Can harmony ever truly stop darkness? Will many years of pain and suffering be unleashed onto Equestria? Only time can tell, but time... is running out. Nightmares overpower dreams, darkness overpowers the light.
It can be difficult writing a present tense story. You seem to remain reasonably true to form. Spelling and grammar are good.
The transition to Nightmare mode seemed very abrupt. In fact, this whole chapter went rather fast. Maybe slow down a little and use more detail?
2699220 I know. I usually just write an okay story that doesn`t have much to it and then go back and make it a great one later. I wrote this pretty fast anyways.