Mirror of Twilight

The Mirror of Twilight is one of the two main quest items in Twilight Princess, along with the Fused Shadows. It is located in the northern section of the Gerudo Desert in a place aptly known as the Mirror Chamber, which can only be accessed by passing through the Arbiter's Grounds.

Background History
Prior to the events of Twilight Princess, the Golden Goddesses banished the Twili out of Hyrule and into the Twilight Realm through the Mirror of Twilight for trying to establish dominion over the Sacred Realm in an event known as the Interloper War. Although the goddesses forbade the Twili to return to Hyrule, the Mirror of Twilight was left as a potential link between Hyrule and the Twilight Realm. The Hyruleans in turn used the mirror as a means to punish the worst criminals in the kingdom: since the mirror was kept in a prison in the Gerudo Desert, the criminals who were sentenced to death were sent away to the Twilight Realm using the power of the Mirror of Twilight. This means of punishment was used by the Sages when the execution of Ganondorf went awry. After Ganondorf murdered the Sage of Water, one of the Sages activated the Mirror of Twilight and used it to send Ganondorf to the Twilight Realm. At that point the Sages were instructed by the goddesses to protect the Mirror of Twilight.

Composition and Functionality
The Mirror of Twilight was made so that only the true ruler of the Twilight Realm would be able to completely shatter it. Should an usurper try to destroy the mirror, he would only be able to break it into shards, thus proving the false kingship.

The mirror functions along with a black stone otherwise known as the Portal Stone. The mirror itself acts as a kind of projector that emits light into the Portal Stone, which activates the portal to the Twilight Realm. When a person steps on the plate in front of the Mirror of Twilight, it will create a set of stairways that will allow said person to walk up to the circular platform that will warp them to the Twilight Realm with the help of the light transmitted by the mirror.

Quest for the Mirror Chamber
The Mirror of Twilight is first mentioned in the game when Link as a wolf takes the gravely injured Midna to Princess Zelda at Hyrule Castle. Thinking that she is about to pass away, Midna asks that Princess Zelda tell Link where to find the Mirror of Twilight. Midna, after having been saved by Zelda's powers, asks Link to help her find the Mirror of Twilight upon reaching the Gerudo Desert. She explains that the mirror is their last link to enter the Twilight Realm and defeat Zant. However, once they reach the Mirror Chamber after having defeated Stallord in the Arbiter's Grounds, they find that the Mirror of Twilight and the Portal Stone are nowhere to be found. After riding atop the statue and using the Spinner at the apex of the large structure, the statue itself dips into the sand, raising not only the black stone, but the Mirror of Twilight itself. Midna, however, realizes that only one fourth of the Mirror is intact, and it is at this point that the Sages make their appearance and explain that Zant has fragmented the mirror with the help of Ganondorf's "mighty magic", not having been able to completely shatter it due to him being the false king of the Twilight Realm.

Quest for the Mirror Shards
Learning that the mirror has been broken in pieces, the pair is unable to enter the Twilight Realm, but the Sages inform them of the location of the remaining three mirror shards: Snowpeak, Sacred Grove, and the City in the Sky.

Once Link and Midna succeed in finding the three missing Mirror Shards, they place them in the Mirror of Twilight and transport themselves to the Twilight Realm, where they ultimately defeat Zant, the Usurper King. After defeating Ganondorf, Link, Midna and Princess Zelda gather around the Mirror Chamber to bid farewell to the Twilight Princess as she returns to her realm. Midna mentions that as long as the mirror is around, they could always meet again, saying that it was the goddess's "design" that they should meet. However, just as she is about to depart, she sheds a tear and uses it to completely shatter the Mirror of Twilight, thus destroying the only known connection between Hyrule and the Twilight Realm.

Despite the fact that the mirror is utterly destroyed by Midna, there may be other ways of traveling between the two worlds. Evidence supporting the idea of other methods of travel between worlds is plentiful, the most obvious piece being that Zant is able to travel between worlds without the benefit of the Mirror. There is fan speculation that the Triforce is somehow related to travel between the worlds, as Zant's power was borrowed from the Triforce of Power. Also, only those characters bearing one of the Triforce could reside in the Twilight Realm without becoming a spirit.

Mirror Shard
A Mirror Shard is a piece of the Mirror of Twilight, which Zant broke so that Link and Midna could not reach the Twilight Realm. Only one Mirror Shard is intact in the Mirror of Twilight when Link first visits the Mirror Chamber. The Sages reveal to the young heroes the locations of the remaining shards: one is "in the mountains", which is where the Snowpeak Ruins are; another is in a "ancient grove" which is actually the Temple of Time in the Sacred Grove. The last shard is "in the heavens", meaning the City in the Sky. Link must retrieve all three missing shards and head back to the Mirror Chamber to reassemble it. When the Mirror is repaired, Link is able to enter the Twilight Realm and confront Zant.

The Mirror Shards can cause misfortune to those who happen upon them. Yeta of Snowpeak mentions that ever since Yeto found a Mirror Shard, monsters have started to appear around the mansion and she has fallen sick. The power of the Mirror Shards is enough to completely transform the others around them, much like how the Fused Shadows drastically changed Darbus into Fyrus. This is shown when Yeta shows Link the Mirror Shard that Yeto found: when she looks at the Mirror, she mentions how beautiful it is, but shortly after its powers cause for Yeta to transform into a monster known as Blizzeta.

Trivia

 * According to the original Japanese text, Ganondorf's banishment in the Twilight Realm seems to have been a special case, since there is no mention of the mirror being used in each execution.
 * When the Mirror is first activated, it broadcasts strange shapes and the symbol of the Triforce. At the opening of Link's Crossbow Training, similar shapes appear and twirl around.
 * In Kirby & the Amazing Mirror, Kirby had to assemble a mirror to defeat Dark Mind, much like how Link had to assemble the Mirror of Twilight to defeat Zant.