General
They say you can find immortality in the Coliseum. They're right, you know. Sora doesn't know this, though if he did, he wouldn't have just stopped at defeating Hades at the Hades Cup, he would have held the god at keyblade point and demand that all contracts be nullified and made void.
Sora doesn't know a lot of things, but he knows that with every Heartless vanquished, a heart is released. Of course, it gets mighty depressing knowing that there are that many Heartless, but Sora quickly grows used to this fact and comforts himself with the idea that he's saving people.
In fact, he is delighted each time he enters the Coliseum, thinking that with every new battle, he frees more and more hearts, more and more souls.
He doesn't know that you can find immortality in the Coliseum, of course. Hades is the god of the underworld, and a villain to boot, so he controls each and every life that passes through. He would have controlled Sora's life force as well, except that Sora never signed a contract with him.
It doesn't matter, because Hades secretly rejoices each time Sora defeats the enemy, savoring the screams and howls of despair as he pulls up each and every freed heart from the pit, sealing them away again in their Heartless shells.
They will face Sora again and again, face the keyblade that holds their salvation, but it will not make a single difference.
They will never die.
They can never die.
You can find immortality in the Coliseum. All you need is a feathered pen and Hades' contract.
You can live forever and ever.
And over.