All the past villains return to wrack havoc in Equestria. To fight back and save Equestria, Celestia assigns the Mane 6 on a quest to use the Elements of Harmony to bring back peace.

Rainbow Dash - Iron Pony
Applejack - Captain Equestria
Twilight Sparkle - Thor
Fluttershy - Hulk
Rarity - Black Widow
Pinkie Pie - Hawkeye
Celestia - Nick Fury
Discord - Loki

Credits to Zaeinn
http://browse.deviantart.com/?qh=&section=&global=1&q=library+ruins#/d4fd55e