The Mortal Mirror was a mirror created by the Darksmith Collective in memory of the Mortal Monks. It was also created as a trap for Manfred Grieg. 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.
When somebody watched as the person punished was consumed, they were 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. It seemed to chase her and follow her. (PROSE: Martha in the Mirror)