Creature07

I'd bet Benjamin that it's the MoM's heart. Really think about how heartless, and more especially, nobodies are created. The MoM fades away, he says so himself at the beginning. "I'm going to fade away," he says. He doesn't die. When the heart leaves the body, a heartless is born from the heart, consumed by the darkness. The body "fades away" and a nobody reappears somewhere else. But in order for a nobody to be created, a heartless must be created with it. It's all in the Ansem reports. So why didn't the MoM become a heartless and form a nobody? Thaaaaat's the whole purpose of the box. It's black, like the cloaks. And any KH fan worth his/her salt knows what the cloaks are for. The MoM used the box to protect his heart from the darkness, thus keeping himself from becoming a heartless. And since no heartless was created when his heart was removed, no nobody reappeared anywhere else when his body faded away. And what's inside that box is another key... THE key to resurrecting the MoM, which we will probably see in KH3.

It's his heart, I'm sure of it. Think about it. The MoM "fades away," he doesn't die. In the KH series, when does someone ever fade away? When the heart is removed from the body. Then the MoM put his own heart into the box to protect it from darkness, and thus, keep himself from becoming a heartless. And since no heartless was created when his heart was removed, no nobody was created when his body faded away. I think he did it to teleport himself into the future, and survive the keyblade war, to fulfill a certain role at a certain time, which he foresaw with the eye in the keyblade. Remember, no one knows what's on that missing page, except Luxu. And he is the secret keeper who the MoM whispered the contents of the box to. We just might see the MoM resurrected in KH3.