The Engulfing Darkness

I assume that you have read Rising Moon if you haven't well read that story first. In that story you will find one reason why Princess Luna had turn into Nightmare Moon and you will find another reason in this story. It all Began the first night Luna had come back after Nightmare Moon had been defeated.

Celestia says "Oh it's your first night back!? This so exciting! There are some many changes and so many ponies to meet! Aren't you Excited!?"

But Luna was restless confused by her jumbled memories, she searched for answer. But isn't prepared for what she had found...

Celestia says "Luna?"

The book Luna was reading slammed shut and Luna jumped.

Celestia says "It wasn't your fault."
Luna says "How can you say that!? After all i have done how can any pony ever forgive me. How can you forgive me!? I don't know what came over me.. how did it happen!? Why!?"
Celestia says "Luna come here and sit i want to tell you a story."
Luna says "I'm not a little filly and your not even listening to me."
Celestia says "just sit and listen. There once lived two sister that cared for each other very much. The oldest brought the beautiful day and the youngest ruled the magical night together they brought balance throughout the land. But as time grew darkness was welcomed into the youngest heart. It wasn't long before ponies of all kinds discovered the wonder of the night. Some started to love the night. The love that came from her subjects and the support she got from her sister, the youngest was happy."