Vaati the Wind Mage in his most common appearance is a black orb with a single eye, though he was originally a human-shaped being. In Four Swords, Vaati is initially sealed in the "Four Sword" weapon, but escapes and captures Princess Zelda, only to be returned to the sword by Link, who is split into four by the sword. In the sequel, Four Swords Adventures, he escapes with Princess Zelda yet again with the help of Ganon, forcing Link to be split up and defeat him. Before the events of The Minish Cap, Vaati was a Picori that became corrupted by the evil in the hearts of mankind. He used the Wishing Cap created by his master, Ezlo, to transform himself into a Hylian sorcerer, and in the game he begins seeking out the power of the Light Force. He eventually becomes his black orb form in this game.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.