Once an Innocent Pony Sunny Moon just wanted nothing but to help her world & make it better place but after facing life & ridicule without having a cutie mark for years & consider a freak among her own family she cast out & completely forgotten as she never existed
For this she now wanted nothing more then revenge on those she once loved & after seeking & finding her in the warmth & comforting arms of Nightmare Moon she was forever changed into a pony of darkness
Sunny Moon became Night Eclipse
She became so dependent on Nightmare Moon learning how to harness her powers & abilities she even began to love her dearly
A love she never thought she get to experience again
Sadly all good things came to an end to Night Eclipse when she found out Nightmare Moon was only training her to use her as a Weapon against her sister Princess Celestia & this revelation finally shatter Night Eclipse mind
Night Eclipse attacked Nightmare Moon with such Fury & Anger that Nightmare Moon was easily bested
Before she was almost destroyed she caught Night Eclipse off guard & placed magical bracelets on her legs which greatly drained her power & kept her from using her full strength
Soon after Nightmare Moon immediately imprisoned her in a dark prison that she would never be able to escape out of as long nothing happen to Nightmare Moon
However when Nightmare Moon was defeated & was return back to her normal self Princess Luna the dark prison she created dispelled & Night Eclipse was Free
Now with the sole desire to Destroyed everything that Princess Luna loves & make her regret for all the years she spent in the dark prison & to remove the magical bracelets that keep her from using her powers fully

So what you Guys think?