Malladus the Demon King was sealed away by the spirits when he terrorized the first settlers of New Hyrule with the Lokomo serving to maintain the seal around the Tower of Spirits. Through the aid of his lacky Cole, who posed as a chancellor to the New Hyrulian royalty, Malladus manages to break free from his bonds and enter the souless body of Zelda. However, when Zelda gets her body back, Malladus is forced to devour Cole and turn him into a new demonic body for him to inhibit. Though the possession would not last long, Malladus intended to destroy everything before being rejected from Cole's body. However, Link and Zelda defeat Malladus with the Lokomo Sword.
Source: Wikipedia, "Characters in The Legend of Zelda series," available under the CC-BY-SA License.