Demise

"Extraordinary. You stand as a paragon of your kind, human. You fight like no human or demon I have ever known. 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 the Skyward Sword. Long ago he and his horde of demons and creatures broke through a fissure in the earth and sought out the Triforce. However, he was defeated and sealed away by the goddess Hylia.

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 through the efforts of Link and Groose. At last, when Link obtains the Triforce, he manages to defeat Demise permanently. However, Ghirahim kidnaps Zelda and takes her to the past through the Gate of Time, 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. Demise begins to absorb the soul of Zelda to gain unstoppable power, but he is impressed by Link's willpower 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 a manifestation 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 will praise him for his courage, but then will tell him that when he falls, he shall rule over everything in the world. With that, the battle begins. Noticeably, he walks towards you, though this isn't important. If you managed to get the Hylian Shield from the Thunder Dragon, you can quickly Shield Attack when he attacks you then follow up ith a vertical attack, then repeat. Eventually, he will be thrown backwards, seemingly defeated, but he will instead rise up, and charge his sword with electricity. Since you will be damaged if you touch his sword, you must point the Master Sword up at the sky and charge it with electricity as well. Use it in a Skyward strike to paralyze Demise, and thrash him around a bit. After doing this a few times, he will fall back again, allowing you to finish him. If you're quick enough and land the attack, the battle ends.

Trivia

 * The sword he holds is a dark version of the Master Sword. It was pulled from Ghirahim's chest, and appears to contain the remains of his soul. At the end of the battle, it fades away to nothing before Demise begins to crumble.
 * His fighting style is similair to Ghirahim's, except his is much finer.
 * He will stare at the Bug Net if you bring it out, similar to Ganondorf staring at the Fishing Rod in Twilight Princess.