A 7 part project detailing the history of Equestria's forgotten past.
As we saw in the return of Nightmare Moon, the very stars themselves would aid in her escape.
But what, or rather who, were those stars.
How did Celestia harness all six Elements of Harmony herself alone?
Why was Luna banished for 1000 years instead of being cleansed like all the others.
These questions and more are answered in Occultus Canonicus, hidden legacy.

Credit to http://dashermkii.deviantart.com/ for the story image.