Mortal Mirror

The Mortal Mirror was a mirror-like object that was created by the Darksmiths of Karagula, (PROSE: The Colour of Darkness) in memory of the Mortal Monks, as a trap for Manfred Grieg.

It reflected a darker but almost identical realm (or dimension) and could trap people in that dimension. If a person trapped in the mirror saw their reflection, their DNA was replaced with one made of a translucent matrix based on silicon, essentially making a person's body living glass.

Governor Pennard, the governor of Castle Extremis, gave the mirror to Grieg as a reward for strategising the recapture of Castle Extremis. However, it was actually a trap set by Pennard, who believed that Grieg was the only one who could overthrow the Third Occupation. He was consumed by the mirror and as a punishment, when leaving the mirror, he would be fragile because he would be made of glass.

General Orlo later claimed to have created a duplicate of the mirror, but it was actually the original, ready to continue with his plan. Martha entered the mirror at one point and found Zerugians asleep in a sort of dormitory. Everyone there were warriors. The Tenth Doctor saved her. When somebody watched as the person punished was consumed, they were officially trapped; hence, the Doctor and Martha were forced to close their eyes as they exited the mirror. Otherwise they would have perpetual glass bodies.

Tylda, originally thought to be Janna, believed that she had a sister, but it was actually her reflection, a mirror image that was created by the Mortal Mirror. It seemed to chase her and follow her. (PROSE: Martha in the Mirror)

The Doctor noticed it was not a real mirror because he would have seen the little girl with the red balloon, who was trapped in every mirror. (TV: The Family of Blood, PROSE: Martha in the Mirror)