Dark Mirror

The Dark Mirror makes its first and only appearance in Four Swords Adventures According to the White Maiden, it is said to reveal the evil that lies within a person, bringing it to life. This is true, as the Dark Mirror used to spawn the Shadow Links which plagues Hyrule and Link throughout the game.

History
When first mentioned by Dampé, the mirror is revealed to have sealed away a dark tribe a long time ago. The Mirror was then hidden away in a temple in the Forest of Light, in order to seal it away from the world.

Upon inspecting the Temple of Darkness, it comes to Links and the maidens knowledge that the mirror had been stolen. They believe it to be Vaati's doing, but clues of the culprit, Deku Scrubs chanting the name Ganon, brings them to the desert, and their suspicion is confirmed by the Red Knight.

Upon arriving in The Dark Cloud of the Realm of the Heavens in the final clash against Shadow Link, Link and Zelda finally manage to retrieve the mirror. This in turn leads to the end of Shadow Link's havoc in Hyrule.

Connections to the Mirror of Twilight
The Dark Mirror has some elements in common with a more famous mirror, namely the Mirror of Twilight. Both mirrors sealed away a dark tribe in the ancient days. There is also some similarity in the evil inducing nature the mirrors has on its environment. While the Dark Mirror spawns evil Shadow Links, the Mirror of Twilight seems to possess people and things, like Yeta.

There is also an interesting allusion in Twilight Princess, where Shadow Links makes an cameo apperance as members of the Dark Tribe in Lanaryu's tale of them.

The appearance of the two mirrors as round with a golden frame is also a similar attribute, although it is hard to ascertain just how similar their physical appearance are, mainly because of the graphical differences between Four Swords Adventures and Twilight Princess.

As for whether the mirrors were actually meant portray the same mirror, is a matter of much dispute in the Zelda communities. Some deem it impossible, as Midna in Twilight Princess destroys the Mirror of Twilight at the end of the game. Others sees this as evidence that Four Sword Adventures does not occur after the Twilight Princess.