Demon

The Demons are a recurring race in the Legend of Zelda series. Demons are monstrous entities who typically have malicious tendencies and hatred of humans, though this is not the case of all demons. Though other demonic entities (Bellum, Majora, etc.) have appeared throughout the series, they have not been officially confirmed as demons themselves.

Biology
Though most appear humanoid, demons often possess monstrous physical attributes; for example, Chancellor Cole possesses horns, and Batreaux appears to be a humanoid bat. Demons can appear in a variety of different forms.

Government
Demons appear to follow a monarchial government, due to the presence of a "Demon King"; Malladus for Spirit Tracks, and Demise for Skyward Sword.

The Legend of Zelda: Spirit Tracks
One hundred years before the start of the game, the Demon King, Malladus, led his demonic hoards in a war against the first settlers of New Hyrule, but the Hylians ultimately prevailed through the intervention of the Spirits of Good, who sealed Malladus beneath the ground through mystical shackles, using the Tower of Spirits as a lock and leaving the Lokomo to watch over New Hyrule as they returned to the heavens. Over the years, the shackles became known as the Spirit Tracks and were used as railroad tracks, and the war eventually became little more than a legend among the people.

In the present day, one of Malladus's remaining minions, Cole, worked undercover as the chancellor of New Hyrule while secretly plotting to destroy the Spirit Tracks and resurrect his master with the help of Byrne, a rogue Lokomo. He eventually succeeded in ridding the kingdom of the Spirit Tracks, and in a confrontation with Link and Princess Zelda, the latter of whom had become suspicious of him, used dark magic to separate Zelda's spirit from her body and stole her body, intending to use it to resurrect Malladus.

However, Zelda survived as a ghost, invisible and inaudible to all but Link and the Lokomos, and together with Link, managed to restore the Spirit Tracks and the Tower of Spirits, though they were too late to stop the ritual, and Malladus was reborn within Zelda's body. Byrne subsequently asked Malladus for the power he had always wanted, but Malladus instead attacked him before departing with Cole on the Demon Train, a massive sentient locomotive, to wait for Malladus to adjust to Zelda's body.

Link and Zelda, with help from Byrne and Anjean, eventually managed to track Malladus down in the Dark Realm, where they used Light Arrows to exorcise Malladus from Zelda's body, which Zelda proceeded to reclaim with the help of Byrne. In retaliation, Malladus killed Byrne and possessed Cole instead; though Cole's body began to reject Malladus's spirit, Malladus vowed to destroy the world before that happened; however, Link and Zelda successfully managed to destroy both Malladus and Cole for good.

The Legend of Zelda: Spirit Tracks
Eons before the beginning of the game, the Demon King Demise led his armies in a bloody war to obtain the Triforce; in response, the goddess Hylia sent the remaining humans into the heavens upon a massive land mass before uniting the Ancient Robots, the Gorons, the Kikwis, the Mogmas and the Parella against Demise's forces, sealing away the demon king within the Sealed Grounds. However, knowing that Demise's followers would free him, Hylia had the Goddess Sword created, discarding her divinity to be reborn upon the residents of Skyloft.

One such servant, Ghirahim, later revealed to be the embodiment of Demise's sword, attempted to kidnap Zelda, the human reincarnation of Hylia, with the intent to use her soul and life force to free Demise from his prison; though Link eventually succeeded in obtaining the Triforce and using it to destroy Demise, Ghirahim simply captured Zelda and traveled back in time to resurrect his master, succeeding in doing so despite Link's best efforts to stop him. Upon his return, Demise, both surprised and impressed to see a human who was not afraid of him, agreed to a final duel with Link, during which he was ultimately defeated. However, in his dying breaths, Demise cursed the bloodlines of Link and Zelda, declaring that an embodiment of his hatred would be reborn and torment the descendants of the two for all time.