Malladus

""In the beginning, the people followed the spirits of good, and all was peaceful. But that era of peace soon came to an end. The evil Demon King rose to power, destroying everything in his path.""

He is already mentioned in the prolouge of the game Spirit Tracks as being the Demon King. When he was sealed into the ground by a set of railroad tracks known as the Spirit Tracks by the spirits of the land, he waited for his chance to be free again. The tracks are referred to as the king's shackles. In the beginning of the game at some point Zelda is killed in order to provide a vessel for the king to walk around in(that sounds awfully familiar). The ones who killed her were her trusted advisor Chancellor Cole and another person who was apparently not human named Byrne. At some point in the game Malladus is revived in Zelda's body which then makes Byrne demand that he get the power that he desires for freeing the demon but he is instead attacked by the demon king who then escapes to the Dark Realm with Cole on a train of his own called the Demon Train. After this Byrne decides to try to instead destroy the demon king along with a Phantom Guardian that Zelda's ghost had possessed.