Mirrorlings were malevolent beings native to another dimension. On one unnamed planet, they used mirrors to assume the appearances of those who looked into them, then switch places, trapping their victims in the mirrorlings' dimension. People on the planet realised that people were being replaced, and discovering that breaking the mirrors would only kill the person replaced, they instead collected all the mirrors and placed them in a nearby castle.
When the Tenth Doctor and Rose Tyler visited the planet, Rose was replaced by a Mirrorling. When the people came to warn him about the mirrors, the Doctor realised that Rose had been replaced. Entering the castle, he lured the Mirrorlings by offering himself as bait, then used his sonic screwdriver to stop them when they were halfway through the mirrors, creating a bridge between his dimension and theirs. He then called out to Rose and the other imprisoned people to escape, and when they had all done so, he used the screwdriver to smash the mirrors, returning the Mirrorlings to their own dimension. (COMIC: Mirror Image)