Mortal Mirror

The Mortal Mirror was a mirror-like object that was created by the Darksmiths of Karagula. It reflected a darker but almost identical realm (or dimension and was able to trap people in that dimension. If a person trapped in the mirror saw their reflection, their DNA was replaced with one made of a a translucent matrix based on the silicon, essentially making a person's body living glass.  (NSA: Martha in the Mirror)

Governor Pennard, the governor of Castle Extremis, gave the mirror to Grieg, as a reward for strategizing the sucessful recapture of Castle Extremis. However, it was actually a trap that was set by Pennard, who believed that Grieg was the only one who could be able to overthrow the Third Occupation. (NSA: Martha in the Mirror)


 * It was not a real mirror, as noticed by the Doctor, because he would have seen the little girl with the red balloon, who was trapped in every mirror. DW: The Family of Blood