Celestia's 'NightmareMoon' (MLP fanfic w/ocs)

(When I get a chance I'll be rebooting this story as well!) You all know Luna was once NightMare Moon. But do you know the story of the time when Celestia became her own version of NightMare Moon? Long ago when they first became Princess she showed the monster who tried to burn the whole planet of ponies. What is the story of the monster, who would watch millions of ponies burn at her hooves?