Mortal Mirror

The Mortal Mirror was a mirror created in memory of the Mortal Monks. It was also created as a trap for Manfred Grieg, by the Zerugians. 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. That was his curse.

General Orlo 'claimed' to have created a duplicate of the mirror, when 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 are officially trapped. 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, which seemed to chase her and follow her. (NSA:  Martha in the Mirror)