Rainbow Dash has experienced many crashes, and somehow manages to pick herself up and carry on after a couple of weeks. However when she suffers a crash that would be fatal to any other normal pegasus and somehow manages to survive,and a suspicious Twilight begins to question how Rainbow is still living, Dash starts to have repetitive nightmares and discovers a terrifying secret that she's been keeping from herself. What and who is the thing inside Rainbow Dash?
A sister?
A protector?
An impostor?
A possessor?
And more importantly; What does it want?

She is Loyalty. She will be the last one standing by her friends, in peril, torture and death.

This is what everyone believes. This is what Twilight and the other Spirits of Harmony, Rainbow's friends, believe. And they took it entirely for granted. After the Mare-do-well incident, Rainbow Dash's Loyalty began to falter. She still felt the terrible sorrow of being deserted by everypony, to be forgotten in the shadow of a greater hero.

Over time sorrow grew to anger. As the powerful bond of friendship crumbles as Rainbow begins to drift away, The others discover just how powerful the Element of Loyalty is, and how its absence might have caused the downfall of the greatest heroes Equestria has ever known.

Rainbow Dash is the hero of the town. She has saved many ponies, and she is very, very proud! But what happens when...

Another hero emerges?

Dash soon learns that this new hero, "Mare Do Well" is really her friends, and this upsets her. Could they really have betrayed her like that?
I don't own the picture.
(based off of Mysterious Mare Do Well) Enjoy:)