Malladus

Malladus, also known as the Demon King, is the name of the primary antagonist of Spirit Tracks. He is a powerful demon that once terrorized the land that would eventually be home to the new kingdom of Hyrule, but was locked away beneath the altar of the Tower of Spirits by the Spirits of Good of the land after a prolonged war between them. In Spirit Tracks, the kingdom's chancellor, Cole, seeks to release Malladus from his prison and help him return to his former strength.

Spirit Tracks
Malladus makes his debut in Spirit Tracks as an extremely powerful demon that once terrorized the land and its first settlers. Over a century later, Hyrule's Chancellor Cole develops a scheme to free Malladus from his prison and unleash him upon the land once again. To do this, he causes large sections of the Spirit Tracks, which serve as the locks to his prison, to begin to vanish. This event causes alarm and suspicion with the land's monarch, Princess Zelda.

After Link defeats Byrne on the top of the Tower of Spirits, he rushes up to the Altar of the Demon King where Malladus had just started taking control of Zelda's body atop the Tower of Spirits. After which, Byrne demands that Malladus give him a gift of power for his role in the recovery; however, Malladus only attacks him, knocking him out cold. As Malladus has not yet acclimated to his new body, though, this exertion is all that he can handle at the moment; he escapes with Cole on the Demon Train, retreating to the Dark Realm.

After retrieving the Bow of Light and the Compass of Light, Link, Zelda, Byrne, and Anjean travel to the Dark Realm to confront Malladus and Cole. Link manages to damage the Demon Train, allowing himself and Zelda to climb on top of it. There, Malladus completes his resurrection, taking total control of Zelda's body. Link and Zelda then fight their way through Malladus and Cole's attacks to the front of the train, where Zelda seizes Malladus; Link then uses the Bow of Light to banish Malladus from Zelda's body. In the ensuing chaos, the Demon Train is destroyed, and all of them are thrown out of the Dark Realm.

Amidst the wreckage of the destroyed Demon Train, Malladus is forced out of Zelda's body. Zelda tries to reclaim her physical form, but finds that for some reason she can't; Malladus tries to repossess her, but he is stopped by Byrne, who gives Zelda the strength to reenter her body. As revenge, Malladus kills Byrne, to Cole's applause. At this point, Malladus possesses Cole instead, transforming him into an immense beast; Malladus is unable to maintain his hold on Cole's body for long, but as he says, he has enough time to reduce the world to ruin. After many long moments, Malladus is finally defeated by Link and Zelda and he and his vessel, Cole, are both destroyed.

Strategy
In the battle, Link defends Princess Zelda from Malladus's fireball attacks. After which, she channels her powers with the Spirit Flute's and her and Link create a duet with her voice and the flute. This reveals Malladus's true weak point which could only be harmed by the Bow of Light. Link must then distract Malladus and have Zelda aim at his back. If done correctly, Zelda will fire one arrow into his back, causing damage. After three shots, a gem forms on Malladus's fore head and Link must strike at it and Malladus's horns. After another three shots, Link must complete the same thing again, destroying the other horn. A final arrow into Malladus's back will force him down and Link must drive the sword clear through Malladus's skull. Zelda will then rush to his side and push the blade more into his skull as well as channel some of her sacred power into Malladus to finally finish off the demon's spirit and destroy his vessel, Cole.

Trivia

 * In the final battle, Malladus bears a strong resemblance to Ganon's Dark Beast form from Twilight Princess, as well the color scheme of his The Legend of Zelda appearance.
 * The Spanish-language version of Spirit Tracks refers to him as "Mallard", which is the name of the fastest steam-powered train ever built, and is also the derivative name for the English one.
 * Malladus is the second villain to possess Zelda's body, the first having been Ganondorf in Twilight Princess.
 * The song that plays as Malladus's Theme after you and Zelda finish the duet is a remix of the duet and the the song that plays when you travel by rails.
 * Since he never leaves a spirit form or body, their is no way of telling what he truly looks like.