I had a mirror of opposition in a game once. The actual mirror had been shattered by another group of adventurers, but afterward pieces began to surface as single-use weapons of desperation. One of the pieces fell into the hands of an enemy of the PCs, a young dragon, and tried to use it to kill them. Unfortunately for the dragon there was an effect in place that prevented the mirror from taking effect- I don't remember what it was, but it was something the PCs used just by chance.
The dragon had to get close to use the mirror shard, so at this point, the PCs were able to finish it off, wrap the mirror shard in cloth and add it to their arsenal. I hadn't planned for it at all, but it was pretty interesting to think that the PCs could now essentially kill any one creature in the world, no matter how powerful. As it turned out, it just sat in someone's bag over many levels and seemed like it would never be used.
The final confrontation of the game had the PCs storming the castle of a possessed queen. The fight with the queen and her guards did not go so well, so with one PC dead and everyone else in a bad way, one of the PCs finally broke out the mirror shard and showed it to the queen. The queen who was benign and good-aligned, a friend to the PCs except for being possessed. So her opposite materialized, and the PCs were now facing two evil queens.
The PCs were able to think fast and take out the duplicate by focusing on the queen. But for an awful moment, it really looked like a TPK.