[Sunset Shimmer]
Power
Was all I desired (I desired, I desired)
But all that grew inside me
Was a darkness I acquired
When I began to fall
And I lost the path ahead
That's when your friendship found me
And it lifted me instead
Like the phoenix burning bright
In the sky
I'll show there's another side to me
You can't deny
I may not know what the future holds
But hear me when I say
That my past does not define me
'Cause my past is not today
Ambition
Is what I believed
Would be the only way
To set me free
But when it disappeared
And I found myself alone
That's when you came and guided me
And it felt like I was home
Like the phoenix burning bright
In the sky
I'll show there's another side to me
You can't deny
I may not know what the future holds
But hear me when I say
That my past does not define me
'Cause my past is not today

she was the personal student of Princess Celestia, before Twilight Sparkle (the central main character of MLP:FiM.)  She was extremely talented, but eventually began to want too much, too fast.  When Princess Celestia refused to accelerate her magic training further, or turn her into an alicorn as she asked (an alicorn is basically a larger, winged unicorn; a pony that possesses the abilities of all three of the main types of ponies), Sunset began secretly studying forbidden dark magic on her own.  When Celestia discovered this and dismissed Sunset from her position as her student, Sunset fled through a magical mirror that led to the human world. 
She returned several years later to steal Twilight's Element of Harmony (a set of six ancient and powerful artifacts.  Twilight's is the central on, the Element of Magic.)  Sunset eventually used the power of the Element to turn herself into a demon, mind controlled the population of a nearby high school, and planned to turn them to demons as well, and use them to conquer Equestria. 
Twilight had pursued Sunset through the mirror, and in the process of trying to locate and reclaim the Element, befriended the human counterparts of her pony friends.  When Sunset tried to blast Twilight with the power of the stolen Element, her friends interposed themselves to try to protect her, which purified the corrupted magic, and turned it back on Sunset Shimmer.