Demise
The Imprisoned
Standing at 50 feet tall, The Imprisoned is the first form of the final boss, Demise. It is a large, black, and hairy monster in Skyward Sword. It is imprisoned (hence the name) in the Sealed Grounds, located right outside Faron Woods. Link first sees this monster in his dreams,
and this monster is the monster that eats Zelda.
Battles
Link appears to have some sort of fight with The Imprisoned in the sealed grounds and it is possible he will be fought again.
Demise
Demise is the Final Boss of the game, and the final form of The Imprisoned, and is the master of Demon Lord Girahim. Girahim successfully revives him in the World of the Past, and Link must defeat him.
Battle
It is not currently known how to defeat demise. The only hint there is is a quote by Fi; "wait until he lifts his sword."
Connection to Ganon/Ganondorf
After he is defeated, he vows to come back time and time again, and to be revieved as long as Hyrule is in existence, possibly hinting that he is the origin of Ganondorf/Ganon