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Oni Link is a form that Link can take in The Legend of Zelda: Majora's Mask using the Fierce Deity Mask. It possesses incredible amounts of power. He is only able to use this mask during boss fights, without the abuse of a glitch.
When Link, who is a young child in The Legend of Zelda: Majora's Mask, puts on the Fierce Deity Mask, his clothes, hair and eyes turn white. He acquires armor and resembles the adult Link seen in Ocarina of Time. His sword intertwines at two different points, the midsection and the top-most part of the sword.