Twilight Realm

"Some call our realm a world of shadows, but that makes it sound so unpleasant...The twilight there holds a serene beauty...You have seen it yourself as the sun sets on this world. Bathed in that light, all the people were pure and gentle...But things changed once that foul power pervaded the world..."

- Midna

The Twilight Realm is the name of a parallel dimension to the realm of Hyrule, a world of shadows and perpetual twilight whose only source of light comes from the two sacred orbs known as the Sols that are kept in a special shrine near the plaza of the Palace of Twilight. It is the homeworld of the Twili, a race of shadow-beings descended from the Dark Interlopers that were banished from Hyrule to this strange new world as punishment for their attempted conquest of the Sacred Realm in an attempt to seize the Triforce. In the aftermath of the Interlopers' banishment, they evolved into the Twili race. The only link between the Twilight Realm and Hyrule is a relic known as the Mirror of Twilight, which acts as the gateway between the two worlds. While Hyrule resides in the World of Light and is lit by the Sun, the Twilight Realm exists in perpetual Twilight, with the Sols acting as the Twilight Realm's equivalent of the Sun. The Twilight Realm is a gloomy, hazy, yet very serene world. It is often known as a world of shadows, though that makes it seem unpleasant. There is a serene beauty to the Twilight Realm, a beauty which the Hylians have seen every time the Sun sets, at the hour of Twilight.

For much of its early history, the Twilight Realm was a peaceful place, until the King of Evil known as Ganondorf was banished to the realm by the Ancient Sages, where his evil influence would cause a coup of the government and cast the realm into darkness by overthrowing the true Twilight Princess and installing a puppet ruler named Zant into the position of ruler of the Twilight Realm. Zant would go on to seal away the Sols, leaving the Twilight Realm in darkness, and invade the land of Hyrule on Ganondorf's orders in order to merge the two worlds together into a single empire. After the Dark Lord's defeat however, peace was restored, and the Twilight Realm was truly free of his grip. This was made possible by the efforts of the Hyrulean Hero Link and Hyrule's Princess Zelda, before the Mirror of Twilight was shattered by Princess Midna, the Twilight Realm's rightful ruler that was restored to power. This action severed the only known road between the two worlds, destroying the only known means of travel between them to prevent similar events from happening again.

History
The Twilight Realm's full history is incomplete, leaving only a brief bit of its origins and its recent history as the only available information regarding its past.

Origins
At a certain point in the history of the land of Hyrule, a war broke out across the land when a group of shadowy magicians known as the Dark Interlopers learned of the mythical Triforce located within the Sacred Realm and attempted to establish dominion over it to harness the Triforce's power. To this end, the Interlopers crafted an ultimate weapon, the dark relic known as the Fused Shadow, inside which they had vested a vast amount of their magic.

When the Interlopers seemed on the verge of victory in this effort, the Golden Goddesses decided to order the Light Spirits of Hyrule to intervene and seal away the dark magic of the Interlopers to halt the war. The Light Spirits, at the order of the goddesses, shattered the Fused Shadow into four fragments (hiding three of them across Hyrule and leaving the fourth in the hands of the Interlopers) and drove the Interlopers across the land of Hyrule into the twilight. They were then imprisoned within the Twilight Realm, a prison world meant to hold those who displease the goddesses, and lost their ruler as punishment for their greedy actions that sparked the war.

In addition, the Twilight Realm's effect of causing those that dwelled within it for an extended period of time to only be able to exist as shadows outside of it took its toll on the Interlopers, dooming them to this fate as well. Over time, the Interlopers would evolve into the Twili and reestablish their monarchy that would come to rule over the Twilight Realm. The goddesses did not completely sever the Twilight Realm's connection to Hyrule however, for they left a lone link between the two worlds in the hands of the Ancient Sages, the Mirror of Twilight, which could only be shattered by the true ruler of the Twili.

The Banishment of the King of Evil
A long time after the banishment of the Interlopers to the Twilight Realm, the King of Evil Ganondorf attempted to launch an invasion of Hyrule, but he was thwarted by the Royal Family of Hyrule, who had been forewarned by Link, the Hero of Time. As a result, Ganondorf was arrested and sentenced to execution at the Arbiter's Grounds, the home of the Mirror of Twilight. Ganondorf was kept there for a time until the day his sentence was to be carried out in the Mirror Chamber atop the prison. When the Ancient Sages impaled him with the Sword of the Sages however, the Triforce of Power suddenly activated within him and prevented his death. The evil Gerudo soon broke free of his chains, obliterated the Sage of Water, and turned the blade on its owners. As a last resort, the remaining Sages activated the Mirror of Twilight to open the portal to the Twilight Realm and sent Ganondorf through it before the portal closed.

The Rise of Zant
When Midna was selected as Twilight Princess, her adviser Zant became enraged that he was not selected instead. He became severely depressed, and in this state was approached by Ganondorf, who masqueraded as a god. Ganondorf offered him power in exchange for his freedom from the Twilight Realm. Zant used his newfound magical abilities to curse Midna and seized power.

The Invasion of Hyrule
Once Zant had secured power, he sealed away the Sols and transformed his people into Shadow Beasts. He then proceeded to attack the World of Light itself. His armies spread Twilight over Hyrule as they conquered, fulfilling Ganon's wish to merge light and shadow. In this Twilight, the people of Hyrule became like spirits, unaware of what had happened to them. Zant broke the Mirror of Twilight into four shards and scattered them over Hyrule to prevent anyone from entering the Twilight Realm.

The Death of Zant
Midna made her way into the World of Light and met Link, who helped her recover the three missing pieces of the Fused Shadow and the missing shards of the Mirror of Twilight. When Link and Midna finally met Zant face-to-face, Link defeated him, and Midna in her rage utterly destroyed him. Zant was not afraid, however. He was convinced that Ganon would resurrect him.

The Shattering of the Mirror of Twilight
When Link met Ganondorf in battle and defeated him, Midna's true form as the Twilight Princess was restored. As Link and Zelda bade farewell to Midna as she returned to her world, Midna shed a single tear as she left. This tear fell on the mirror and shattered it, severing the link between the two worlds.

Trivia

 * The Twilight Realm originally had a flat, monochromatic look. This early version of the Twilight Realm was featured in several trailers for before it was revamped before release.