Long was the Shadow She Cast

The sister meant to protect her, is the one who drove her into the arms of darkness. Celestia blames herself for everything that turned her sister into Nightmare Moon. Based on Ponyphonic's "Lullaby for a Princess".

Feeling rejected from their subjects and even her own sister, Princess Luna undergoes her transformation into the evil Nightmare Moon. The only pony able to stop her is her own sister Celestia who must banish her in the end.