Demise is the final boss and ultimate antagonist of Skyward Sword. Link first encounters Demise in the Sealed Grounds as The Imprisoned, a primal and incomplete manifestation. Ghirahim is the malevolent counterpart to Fi, acting as Demise's weapon and agent to reawaken him.
After Link fights Ghirahim for the third time and bests him in combat, Zelda/Hylia's spirit is partially absorbed by Demise, giving him just enough power to bring forth his true form. After antagonizing Link, he retreats into a strange portal. Link follows him to defeat him and reclaim Zelda's spirit. During the course of the battle, the Master Sword's Skyward Strike is disabled, putting Link at a disadvantage against Demise. However, Demise's power comes from the lightning striking his blade from the clouds. Link takes advantage of the same power in his fight against Demise and ultimately defeats him, restoring Zelda's spirit and sealing Demise once more.
Before he is completely sealed away, Demise explains that he cannot be defeated and his hatred will simply reincarnate. He further explains that he cast a curse upon the goddess and her chosen hero, forcing them to reincarnate with him through their own bloodlines. This sets the stage for all future Zelda games and explains why there are different Zeldas, Links, and even different villains, such as Vaati and Ganondorf. This new information may also deny Zelda 2 from being the canon ending of the series; instead it may ensure the storyline never ends. More information about the nature of Demise's curse is needed before solid theories can be made.
- During the second half of the fight with Demise, Link can attempt to use the Skyward Strike but instead draws lightning from the sky in a similar way to Demise. This can then be use to temporarily incapacitate Demise, leaving him open for attack.