Demise, the Demon King, also known as The Imprisoned, is the final boss of Legend of Zelda: Skyward Sword, and is the master of the central antagonist, Ghirahim. He is the Anti-God the The Legend of Zelda universe. It is said that he has conquered Time and that he is the source of all demons and monsters. His hatred was reincarnated in the form of Ganondorf, the primary antagonist of the franchise. Being the source of all monsters, he is indirectly responsible for the existence of every evil being that ever threatened Hyrule.
In the beginning of time, after the Golden Goddesses Din, Farore and Nayru shaped the world and returned to Heavens, leaving the Triforce behind, Demise appeared from within the earth. Since he coveted the newly created world for himself, he sought to gain the Triforce and use its power to surpass the Golden Goddesses and pry the world from them. He spawned an army of monsters and invaded the surface, spreading plagues and destruction on his wake and killing millions without mercy.
However, the Goddess Hylia, patron of life and keeper of the Triforce, established mankind (which was powerless against Demise) on floating islands in the sky, where she hid the Triforce and the Goddess Sword: a weapon potent enough to fell the King of Evil. After this, she led the five other races of the world (probably Dekus, Kokiris, Gorons, Zoras and Fairies) in a war against Demise's forces and held them at bay. Hylia eventually sealed Demise away; reducing him to a gigantic monster who would later be known as the Imprisoned. A Sealing Spike marked his seal in the Sealed Grounds, at the bottom of a great pit behind Faron Woods. (However, she was to severely wounded and had to reincarnate herself as a human.)
After Demise's imprisonment, the spirit of his sword, Ghirahim, vowed to free his master. Once the seal began weakening, Ghirahim began to set about reviving Demise using the soul of Zelda, Hylia's reincarnation in human form.
Legend of Zelda: Skyward SwordEdit
After Hylia's servant, Impa, escaped with Zelda into the past, and a Gate of Time was revealed in the Sealed Grounds, the seal on Demise broke and he escaped under his Imprisoned form. However, the Chosen Hero of the Goddess, Link, defeated him, driving the Seal Spike into his skull and resealing him below the Sealed Grounds.
Demise later escaped again, but was again defeated and sealed up by Link, with help from a fellow child of the sky, Groose, and his catapult. This happened a third time as well. As Demise was about to escape a fourth time, Link made a wish with the Triforce, causing the Statue of the Goddess to fall and crush Demise, obliterating him.
However, Ghirahim traveled to the past with Zelda and, by means of a ritual, gave her soul to Demise, who was able to return to power. Demise transformed Ghirahim back into a sword and remarked Link, who is facing him without fear. Favourably impressed, Demise told him that if he wished to fight him, he should enter his own dimension away from any distractions, otherwise, he could just wait for Demise to destroy the entire world.
Link entered the dimension and fought Demise. Eventually, Link defeated Demise by stabbing him with a Killing Blow using the Master Sword. Before disintegrating into black smoke, Demise praised Link's valour but cast a curse so that his hatred would be reincarnated to torment Link and Zelda's descendants forever. What remained of Demise's consciousness was absorbed into the Master Sword, to decay over time until there was nothing left.
When Ganondorf manages to transform into the Demon King Ganon, he regains the power he once had as Demise. Save for its resurrection in the Oracles series, it is also possible that he regains his memories as well. Demise is directly mentioned in Four Swords Adventure by Zelda in the line "Ganon... this beast was once of the Gerudo... Once human. He was called Ganondorf! King of Darkness, ancient demon reborn."