Due to a failure of the ponies who lived before Equestria, a spacial alignment that would normally be nothing, instead opens up a portal to an alternate dimension which is home to a world that's inhabitants are the exact opposites of the ponies living in Equestria. As an attempt to stop the evil ponies within this alternate universe from destroying the wonderful world of Equestria, Celestia sends in an A team to hold down the world for the three years the portal will remain open.

Link for picture: http://browse.deviantart.com/?qh=&section=&q=shadowmares#/d4j9fwu
I did not make it, all credit for the picture goes to aeroyTechyon-X on deviantart.