The mare woke up surrounded by five ponies.

"Ahhhhhhhhhhhhhh! You found out!" screeched the mare, turning into a Changeling. Twilight gasped and fired a laser. But the changeling had advantages. It went straight through a hole in her wing, bounced off a mirror, and hit Rainbow Dash square in the face. An army of Changelings exploded into the castle, breaking many items.

The Mane Six readied for attack. Well, minus Fluttershy.
They fought.

Laser blast!
Fashion Kick!
Party Cannon!
AppleBuck!
Rainbow Punch!

Changeling guts were everywhere.

"I guess it's time to get the mop."

Author's Notes:

Muahahahahahaha, nope I lied.