This fic is based on the Hour of Twilight Sparkle/Cataclysm theory. Well, this is a fic revolving around the entire theory. See the description below.

When Twilight Sparkle begins to notice a strange connection between many old villain's new appearences, she reads up on her theory. An alliance of powerful villains called the Nine Circles of Death are coming together to aid the devil of the MLP:FIM universe known as The Destroyer, The End of All Things, and many other names in ending all mortal existence. Twilight Sparkle and her friends are put to the ultimate test when Mortuus unleashes his greastest creation, the Rainbow of Darkness, on their world. Cataclysm falls as the the end draws nearer... Will the six friends stop this force of darkness from ending all existence?

The teaser trailer can be seen here
Info on the original theory can be found here
Artwork on the theory can be viewed here