Demise

"Though this is not the end. My hate... never perishes. It is born anew in a cycle with no end! I will rise again! Those like you... Those who share the blood of the goddess and the spirit of the hero... They are eternally bound to this curse. An incarnation of my hatred shall ever follow your kind, dooming them to wander a blood-soaked sea of darkness for all time!"

- Demise's final words

Demise is the main antagonist and final boss of SS undefined. Long ago, he and his horde of demons broke through a fissure in the earth and sought out the Triforce. However, he was defeated and sealed away by the goddess Hylia and after the events of Skyward Sword, finally by Link through use of the Master Sword, in both past and present. Demise carries a cocky, overbearing temperament, and will stop at nothing to prove his power to any who defy him, even to Link, a non-magical mortal. Fi describes Demise as "the source of all monsters", and states that he "has conquered time itself". Demise has a strong resemblance to Ganondorf; large stature, fiery red hair, dark skin; even his facial features are similar, and in the cutscene before the final battle with him, a variation of Ganon's leitmotif plays. Upon his defeat, Demise makes a claim that his hatred will not end, but will be reborn in the coming ages. The form of this evil is thought to be Ganon. Hyrule Historia specifically mentions that his flaming red hair is to signify a connection to Ganondorf.

Background
When the gods of old departed from the world, they trusted their ultimate power, the Triforce in the hands of Her Grace to protect it. However, Demise, wanting to make the world his own, gathered an army of monsters and together launched an attack against the people of the surface, murdering them and causing misfortunes all just to get their hands on the power guarded by Hylia.

To prevent the great power from falling into the hands of Demise, Hylia and the five tribes who lived on the surface joined forces and sealed away Demise, restoring peace to the land. Due to the seal placed upon him, he assumes the monstrous form as The Imprisoned for the majority of the plot.

Resurrection
Ghirahim, who is actually Demise's sword, pursues Zelda to steal her soul and thus resurrect his Master, Demise. Meanwhile, Demise attempts to break free of his seal by his own means in the form of The Imprisoned, but he is consistently repelled and re-sealed beneath the Sealed Grounds through the efforts of Link and Groose. At last, when Link obtains the Triforce, he manages to defeat Demise permanently by driving the Statue of the Goddess from Skyloft into the Sealed Grounds as The Imprisoned awakens for the fourth time. However, Ghirahim kidnaps Zelda and takes her to the past through the Gate of Time in the Sealed Temple, where he begins a ritual to revive his master.

While Link manages to defeat Ghirahim, he is too late to stop the resurrection of Demise. The Imprisoned begins to absorb Zelda's soul and eventually transforms into the Demon King himself. Ghirahim welcomes back his master, but Demise pulls a dark sword out of Ghirahim and seals his entity within the weapon. Demise continues to absorb the soul of Zelda to gain unstoppable power; he is impressed with Link's determination to protect the goddess and invites him to do battle. To Demise's shock, Link manages to defeat him, but in his dying throes, he warns Link that it was not over, and that an incarnation of his hatred would be reborn as a being to pursue domination of the world, as well as those with the blood of the goddess and the spirit of the hero, in a cycle without end. With that, Demise is sealed away within the Master Sword, where his remains begin to decay.

Battle
After Link enters the chosen battlefield through the portal in the past, Demise praises him for his courage, but then tells him him that when he falls, he shall rule over everything in the world. With that, the battle begins.

Striking Demise through his guard can be difficult, as he will attempt to block every attack Link makes. Additionally, Link's Skyward Strike is unavailable as the power of the Goddess is negated by Demise's personal battlefield. Link can quickly Shield Attack when Demise attacks with his sword, then follow up with a vertical attack or Spin Attack. Eventually, Demise will be thrown backwards, seemingly defeated, but he will rise up and summon a storm. Demise can then draw the lightning from the storm to charge his sword with electricity. Link will now be damaged if he strikes the sword and Demise will cause twice as much damage as before. To defeat Demise, Link must chose his attacks wisely. Demise is now able to perform his own variation on a Skyward Strike, after which he is forced to recharge his sword by pointing it skyward. It is possible to defeat Demise by attacking him before he can recharge his blade, but it is easier to mimic Demise and point the Master Sword to the sky. This will charge the Master Sword with electricity from lightning bolts, allowing Link to attack with a Skyward Strike and paralyze Demise, making him vulnerable to attack.

After doing this a few times, he will fall back again, allowing Link to finish him with a lightning charged Fatal Blow. Demise is swift however, and he will usually dodge the attack at first, and sometimes the second. If Link is quick enough to land the second attack, or attempts to land the third, the battle ends.

Trivia

 * Demise will stare at the Bug Net if it is brought out. This is the latest in a long line of seemingly-useless "catching" items having an effect in an important fight: deflecting Agahnim's attacks with the Bug Net in ; volleying Ganondorf's attacks back at him with an Empty Bottle in ; and distracting Ganondorf in a similar way with the Fishing Rod in . As with Agahnim's attacks, Demise's Skyward Strike can be reflected by the Bug Net.
 * Demise is known as the Demon King, a title he shares with Malladus and is one of Ganon's many titles. Although it is translated differently in different games, in the Japanese games Ganon's title is commonly, meaning Great Demon King. In the Japanese version of Skyward Sword, Demise is also known as Tyrannical Being and the Being of the End.
 * Despite the fact that Demise has a title, the boss fight does not label it; only his name. This is similar to Ganon in the final battle of.
 * Despite other characters referring to Demise specifically as male, Fi instead refers to Demise as "it". When analyzing him, however, Fi refers to Demise as "he" and "him" again.
 * Demise bears a curious glowing, X-shaped scar on his forehead. This may be the result of the Sealing Spike. Anatomically, the mark's position on Demise's true form is difficult to compare to his form as The Imprisoned, as his head morphs a great deal during this transformation. However, it can be argued that the scar, once hidden by The Imprisoned's numerous "quills", was revealed once his forehead reformed. Curiously, this area of the brow is often occupied by the signature jewel which adorns his successor, Ganondorf. In Twilight Princess, one of Dark Beast Ganon's weak spots is his forehead, which glows in a similar manner. Also in Twilight Princess, a similar "glowing scar" is visible on Ganondorf's chest, where he was penetrated by the Sword of the Sages.

Theory
Some have called attention to the fact that the inverted Triforce insignia on Demise's sword is also the mark of the Lorulean Triforce. This could potentially mean that Demise originated from Lorule. This is further supported by the fact that, according to the prologue of SS undefined, Demise and his armies emerged from a fissure that appeared following an earthquake. This is similar to the fissures that appeared all over Hyrule following the earthquake caused by Yuga in, and Link travels between Hyrule and Lorule through these fissures. If true, this could indicate that Demise was responsible for the crack linking the two worlds in Lorule's Sacred Realm. This connection could also mean that Demise's sword is the Lorule equivalent of the Master Sword, with Ghirahim as the Lorule version of Fi accordingly.