The Loyalty Booster?

A new portal has opened in Equestria, and the changelings intend to conquer a new home. Now it's up to Twilight and her friends to close it before they get the chance. When Rainbow chases a changeling through, she traps herself, Twilight, Spike and the changeling in another world. To get home, she has to team up with her arch enemy and pretend to be the Energy Booster! RDxOC