Dark Mu is a small, fairy-like creature who is capable of easily destroying worlds. Dark Mu is Kris Mu's sister.
After Dark Devil Shredder is defeated by Donatello and April O'Neil, Dark Mu retreats to outer space in order to destroy the Earth. The Turtles fuse together to become Turtle Saint, and use their special move Mega Final Saint Break to seal her with the help of Kris Mu's sacrifice.
Dark Mu is so powerful, that her very presence alone is enough to destroy stars, which would eventually lead to the destruction of an entire Galaxy.