@Background Pony  
Hoorah for plausible fan-stories!

and thus… the story of Twilight Sparkle, the student of Princess Luna began… on that ever lasting day

“All evil shall burn under my righteous gaze…”

“Once upon a time, in the land of Equestria, there were two regal sisters who bring balance and harmony to each other and their subjects, all the different kinds of ponies.
 
The eldest, Princess Celestia, use her powers to bring out the Sun, and rule over the day. Benevolent and compassionate, all Princess Celestia wanted out of anything else was to protect her subjects and all she loved from all harm.
 
But her worst fears was realized for the first time when she was forced to banished her younger sister to the Moon, leaving her alone and guilt-ridden for the next one thousand years.
 
Although her sister eventually returned, and the power of the Elements of Harmony return to her aid with new bearers in the form of her student and her friends, unease and uncertainty remained in her soul. Had she become so weak and pathetic, she was unable to defend Equestria by herself and had to hide behind the Elements?
 
She thought she knew the answer to that when she found herself powerless against chaos spirit Discord, and was outright defeated by Queen Chrysalis of the Changelings. Her confidence in her ability to defend her subjects was all but shattered.
 
Her spirit, weakened by her failures, allowed darkness to find anchor. She came to believe that she had not done enough to protect her subjects, and that it was a mistake to restrain herself and hold herself back, when all it did was making her falter against those that threaten Harmony.
 
Slowly and surely, she let her own power and her one desire to darken her heart, until one day, they consumed her completely.
 
On one fateful evening, she refused to lower the Sun, and she vowed that she would plunge Equestria into eternal day until all she see as evil and impure be destroyed by her power.
 
And thus, with a thundering declaration, she was transformed into a mare of radiant malevolence:
 
“I am Solar Flare! All will be purified by the cleansing fires of my sun!”
 
… “
 
= The Tale of Solar Flare, the Nightmare of Princess Celestia and the Terror of the Day =