The Twilight Realm is a world that once housed the most notorious criminals of the light world, condemned to it by the power of the Sages. After many generations, it's inhabitants adapted to the perpetual twilight and took on beastly forms, referred to as Twili. Eventually, a monarchy was formed as the Twili practiced their magic skills, and became the Twilight Realm as seen in the game.
The Twilight Realm is the native land of both Zant, and your companion, Midna, the true queen of the Twilight Realm. However, when she was overthrown by Zant and cursed to the imp form that you meet her in, both the Twilight Realm and the light world were thrown into chaos.
Sols are the main source of energy in the Twilight Realm. They are protected by large 'hands' in the Palace of Twilight, but must be retrieved and placed on small depressions in the floor to power various platforms, doors, and mechanisms. Sols also repel the dust that transforms you into a wolf. Once you find two sols in the Palace of Twilight and place them together near the entrance to the Twilight Realm, you will be able to house the power of the sols within your sword.
The main method of traveling between the Twilight Realm and the light world is through the Mirror of Twilight at the Desert Colossus. However, Zant has the ability to travel between both worlds using his warping power. This is why he broke the mirror, so that nobody else could travel between the two worlds. However, only the true ruler of the Twili has the ability to destroy the mirror completely, which is why Zant could only break it into pieces.
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