"The Sun offers burning light, while the Moon offers gentle rays. The Sun offers lies, while the Moon offers truth. The Sun ignores the needy, but the Moon raises thee up." Dabria Alicornae
Nightmare Moon won. She defeated Celestia, broke the Elements, banished the sun. This all happened a very long time ago. So long ago, that she has had plenty of time to change her mind. Not that fixing it will be simple.
The war is over, Equestria is at peace once more. But what peace it can hope for with Nightmare Moon now on its throne? Well, perphaps she would prove to be much different than anypony belived.
What would you do if you already had everything? Or everything you wanted. Her Majesty Nightmare Moon has recently discovered there is more to life than war, intrigues and edicts. Luna is trying to be helpful. She has no choice.