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!"

- The curse of Demise

Demise is the main antagonist and final boss of Skyward Sword. 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 temperance, and will stop at nothing to prove his power to any who defy him, even to Link, a non-magical mortal.

Background
Throughout the events of the game, Demise attempts to break free of his seal 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 Lord 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 places a curse on Link and Zelda. For all eternity, they and their descendants shall be pursued by an incarnation of Demise's hatred. 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 and he will often retaliate immediately. 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 the Ghirahim-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, 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 will likely dodge the attack at first. If Link is quick enough to land the attack, the battle ends.

Theory

 * Demise possesses a sword that seems strikingly similar to the Master Sword - it has a similar hilt shape and (upside-down) Triforce engraving. It remains unexplained whether the blade wielded by Her Grace Hylia and her chosen hero was specifically created to counteract Demise's weapon or vice versa. This is made further unclear by the fact that Fi and Ghirahim are similar humanoid beings accompanying each sword.
 * It is possible that Ghirahim is a sword spirit like Fi and the sword that Demise wields is actually the sword of whom Ghirahim is the spirit of.

Trivia

 * Demise will stare at the Bug Net if it is brought out, similar to Ganondorf staring at the Fishing Rod in Twilight Princess.
 * Demise has a strong resemblance to some Ganondorf incarnations in other games. Considering that Ganondorf could be reincarnated from Demise's hatred, this is likely not a coincidence.
 * Demise is commonly compared within the fandom to Akuma from Capcom's Street Fighter franchise. In particular, Demise strongly resembles Oni Akuma from Super Street Fighter IV Arcade edition.