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
In the first phase of the battle, Demise's only attack is slashing with his sword, wich is pretty awkward to avoid, but with good timing and reaction, you should get the job done. When you try to hit Demise with your sword, you shall quickly find out he will block almost any attack, but Fi gives a good hint to defeat Demise:"Wait until he raises his sword.", it's one of the only gaps in Demise's defense. When you have hit him enough, he'll fall unto the ground, when he does so, you'll have to quickly deliver an attack before he gets up. Then you'll start the second phase of the battle, when you don't, he'll get up and you have to start all over again. In the second phase of the battle, he'll summon lightning clouds and he will start to fight serious. His primary attack will still be the same, but this time he'll also drop lightning in order to hurt you. This time, you have to stick your sword in the air when he'll drop lightning in order to absorb it in your sword and slash towards Demise so he'll get stunned and then hit him with your sword. When you hit him enough he'll again fall unto the ground and you'll have to deliver the final blow. When you do so, Demise is defeated.
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