- "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 (影の世界 Kage no Sekai?, Shadow World) is a location from The Legend of Zelda: Twilight Princess. The Twilight Realm is another dimension. The Realm is home to the Twili race and is ruled by a king or queen. The Realm is lighted and protected by the sacred Sols, orbs containing the power of Light. Apparently, it has gods of its own; when the light of the Sols enters the Master Sword, Midna says it is because the gods of the realm have blessed it.
The realm is covered with an abyss of twilight, and has black buildings everywhere that seem to float above nothing despite a clear presence of gravity, giving another hint of advanced technology. However, this may be the magic of the Interlopers put to a better use. Despite the fact that there seems to be many buildings within the realm, the only one Link is able to visit in-game is the Palace of Twilight.
In the early history of Hyrule, a war was fought over the power located in the Sacred Realm, the Triforce. Among the warring factions, a tribe that excelled in dark magic arose and attempted to steal the Triforce. The three goddesses banished these people to the Twilight Realm and sealed away their dark magic. The only link connecting Hyrule and the Twilight Realm was the Mirror of Twilight. The tribe banished to the Twilight Realm eventually became the Twili, and they gradually lost all ambition to conquer the World of Light. Instead, they began making a living for themselves in their new world.
Attack on Hyrule
Zant was a Twili who had long served the royal family, believing he would be chosen as the next ruler of the Twilight Realm, but he was not granted this honor. While Zant was consumed by hatred and despair, Ganondorf's spirit appeared to him, having previously escaped execution at the Arbiter's Grounds and been sent to the Twilight Realm. Ganondorf housed his power in Zant, who took Ganondorf as his new god. Zant gained enormous power and usurped the throne of the Twilight Realm, turning its people into Shadow Beasts. Zant proceeded to invade Hyrule, shrouding the land in Twilight.
Attack on the Palace of Twilight
Link, the hero chosen by the Goddesses, frees Hyrule from the Twilight, and warps himself into the Twilight Realm using the Mirror of Twilight. He restores the power of the Sols to the realm, freeing the Twili from Zant's curse. The power of the Sols is then transferred into the Master Sword, which gives Link the power to blot out the darkness. Link defeats Zant in a final confrontation, and Midna uses the power of the Fused Shadow to kill Zant. However, as long as Ganondorf lives, he can resurrect Zant at any time. When Link finally defeats Ganondorf, Zant can no longer be revived, and is truly dead. Afterward, Midna returns to the Twilight Realm, presumably taking her rightful place as its ruler. She destroys the Mirror of Twilight as she leaves, breaking the connection between the two worlds.
The Legend of Zelda: Breath of the Wild references
The Twilight Realm and related equipment set is mentioned a number of times in the game including during one of the memory sequences, and the gear can be obtained by using the amiibo Rune and the Link, Zelda and Ganondorf amiibos of the Super Smash Bros. series, including the Cap of Twilight, Trousers of Twilight, Tunic of Twilight, Sword of the Six Sages, etc. It is possible to summon the Twilight Princess version of Epona from this Link amiibo as well. By using the Zelda amiibo of this series, it is possible to get the Twilight Bow. The Wolf Link amiibo summons Wolf Link as a companion.
The Twilight Realm appears as part of the Palace of Twilight stage. Despite the many other alternate realms and dimensions that appear throughout the series (some of which are depicted as adventure maps), this is the only one that is depicted as a core gameplay stage. Interestingly, the atmosphere of Twilight Realm does not alter any Light World inhabitants (just as the Twilight Keeps that appear on Twilight Field) and the Palace of Twilight contains a Fairy Fountain where a Great Fairy lives.
It is theorized that the Twilight Realm is what remains after the Moon crashes into Termina in The Legend of Zelda: Majora's Mask. Some believe that the Song of Time does not completely remove the timeline Link travels back from in that game, but instead creates a new one and sends Link into it, as that is what happened when Zelda did this to Link in The Legend of Zelda: Ocarina of Time, meaning that the timeline Link leaves behind will inevitably be destroyed by the moon. Since the Song of Time is required to be played at least once to progress through the game, an ending where Link does not play the Song of Time is impossible.
It should be noted, however, that the means by which the Hero of Time Link leaves the timeline that Wind Waker and its followups exist in is through the limited magic Zelda has gained through both the Triforce of Wisdom and temporarily becoming the seventh sage. The Song of Time is likely a more clinical means of time travel that does not leave a separate timeline behind each time it is used.