Malladus

The Demon King is the name of the primary antagonist of Spirit Tracks. According to details released concerning the game, the Demon King was a tyrannical despot that rose to power and ruled over the new land that was discovered by Link the Hero of Winds, Tetra and the Pirates that eventually became the new land of Hyrule. After his defeat at the hands of the good spirits of the land, who exhausted all of their powers to defeat him, the Demon King was bound and locked away, held in his prison by a special set of chains, a series of train tracks built across the land known as the Spirit Tracks, whose existence depend upon the state of the Tower of Spirits, which acted as a lock to seal him in his prison. These tracks would expand in four directions all across the kingdom of Hyrule and eventually come to be used as a means of transportation by the general populace of Hyrule. With the Demon King's defeat, the good spirits returned to the heavens, and the land he once terrorized was given over to the new settlers, who proclaimed it as the new land of Hyrule, with the Hyrulean monarchy being reestablished to govern the land.

A century later, Princess Zelda comes to suspect that one of her government officials, Chancellor Cole, is involved in a plot to destroy the Spirit Tracks that hold the Demon King in his prison and to revive the ancient evil that once terrorized the land. As the tracks begin to mysteriously vanish across the land, the princess confides her suspicions of the Chancellor in Link, her close friend and the recently-named Royal Engineer to pilot the Spirit Train. Her suspicions later prove to be correct when the Chancellor abducts the princess's body, ripping her spirit from it and intending to use the body of Hyrule's princess to somehow restore the Demon King to his former strength. It is then that Link journeys across Hyrule with Princess Zelda's spirit to restore the Tower of Spirits to its former state, restore Zelda's spirit to her body, defeat the Chancellor, and ultimately stop the return of the wicked Demon King to power.

Spirit Tracks
The Demon King makes his debut in Spirit Tracks as an extremely powerful demon that once terrorized and ruled over the land that was discovered by the Hyrulean settlers that included the Hero of Winds, Tetra and her Pirates. He is shown in the introduction of Spirit Tracks, attacking the land and causing much devastation and destruction to the land. However, it is revealed that the good spirits that resided across the land rebelled against him and used all of their energies to overthrow him, though they did not possess enough strength to truly finish him. Instead, he was buried deep in an underground prison, and a special set of chains were constructed that spanned across the land as well as a great tower that acted as the lock on the Demon King's prison. These came to be known as the Spirit Tracks and the Tower of Spirits, and as long as they remained intact, the Demon King would be safely bound within his prison. After this great effort, the good spirits of the land were exhausted of their power, and so they returned to the heavens, but not before giving the land over to the new settlers, who proclaimed the land as the re-established kingdom of Hyrule and revived the Hyrulean monarchy to govern the land just as it had been with its forbearer.

Over a century later, Hyrule's Chancellor Cole concocts a scheme to free the Demon King from his prison and unleash him upon the land once again. To do this, he causes large sections of the Spirit Tracks to begin to disappear, causing great alarm in the land's monarch, Princess Zelda. Zelda immediately suspects Cole due to his untrustworthy nature and decides to take matters into her own hands by investigating the Tower of Spirits herself without Cole's knowledge. Though she flees with her friend Link aboard a train to the Tower of Spirits, they crash along the way due to the tracks disappearing beneath them. They then witness the destruction of the Tower of Spirits, as it shatters into four segments that are swept away. The Demon King then escapes admist the destruction and takes the form of a demonic train. It is at this moment that Chancellor Cole drops his guise and reveals himself to be a follower of the Demon King, knocking Link and Alfonzo out and stealing Zelda's body, meaning to use it to somehow restore the Demon King to full power. This marks the beginning of a quest for Link to retrive Zelda's body, restore the Tower of Spirits, revive Zelda completely, and ultimately stop the Demon King's return to power.

Trivia

 * The Demon King bears a strong resemblance to Ganondorf, especially in his  The Wind Waker appearance, however it has been confirmed that the relation is aesthetic in nature and that the Demon King is not Ganon.
 * The Spanish-language version of Spirit Tracks refers to him as "Mallard."