I think it contains Xehanort's Guardian. Please read for explanation, though most of the points are speculation I've tried to base them on facts and events that have occurred. Please let me know what you think.
Reason being:
On the box there are 20 locks representing the number pieces necessary to forge the X-Blade, on the cover of the box we have a symbol with 13 points i.e. number of seekers of darkness.
Part 1 of my theory thinks that the box contains the dark counterpart component necessary to reach kingdom hearts (because of the 20 locks and 13 points shown)
Part 2 of my theory believe that Xehanort's Guardian
Specifically I believe whats inside it is a dark heart which becomes Xehanort's Guardian. The is because Xehanort's Guardian is part of 'Ansem Seeker of Darkness' Seeker of Darkness being the term used to describe the dark counter parts to the coming keyblade war. You could say Guardian belonged to the original Seeker of Darkness of their world and from the original other seekers of darkness are being created. Xehanort was passed Luxu's keyblade so he have just as easily passed whats inside the box.
I believe the Xehanort's Guardian is the dark heart given shape. It is associated with Ansem SoD because the Heartless hold the heart so the nobody counter part wouldn't have it. It could be why Ansem kept his human shape when he was a heartless i.e. instead of taking on a monster form like the other heartless the dark heart inside transformed and took shape as the Guardian.
It could explain why there are 13 seekers of darkness but only Ansem is shown to have the Guardian. Although I cannot explain why the Guardian surfaces during the battle between Terranort and Aqua, it could be that because Terra and Xehanort's hearts where conflicting this third party counter part took shape as the Guardian but only Ansem kept that piece because heartless keep the heart and nobodies are empty vessels, only when Xehanort became whole did he try to create copies of himself.
I feel that Xehanort's Guardian is the last loose end that and I think its a nice way to tie the end of KH3 to the next saga, and it perfectly explains Luxu's expression when the Master whispered to him whats inside and possibly why he warned him not to open it.
Reason being:
On the box there are 20 locks representing the number pieces necessary to forge the X-Blade, on the cover of the box we have a symbol with 13 points i.e. number of seekers of darkness.
Part 1 of my theory thinks that the box contains the dark counterpart component necessary to reach kingdom hearts (because of the 20 locks and 13 points shown)
Part 2 of my theory believe that Xehanort's Guardian
Specifically I believe whats inside it is a dark heart which becomes Xehanort's Guardian. The is because Xehanort's Guardian is part of 'Ansem Seeker of Darkness' Seeker of Darkness being the term used to describe the dark counter parts to the coming keyblade war. You could say Guardian belonged to the original Seeker of Darkness of their world and from the original other seekers of darkness are being created. Xehanort was passed Luxu's keyblade so he have just as easily passed whats inside the box.
I believe the Xehanort's Guardian is the dark heart given shape. It is associated with Ansem SoD because the Heartless hold the heart so the nobody counter part wouldn't have it. It could be why Ansem kept his human shape when he was a heartless i.e. instead of taking on a monster form like the other heartless the dark heart inside transformed and took shape as the Guardian.
It could explain why there are 13 seekers of darkness but only Ansem is shown to have the Guardian. Although I cannot explain why the Guardian surfaces during the battle between Terranort and Aqua, it could be that because Terra and Xehanort's hearts where conflicting this third party counter part took shape as the Guardian but only Ansem kept that piece because heartless keep the heart and nobodies are empty vessels, only when Xehanort became whole did he try to create copies of himself.
I feel that Xehanort's Guardian is the last loose end that and I think its a nice way to tie the end of KH3 to the next saga, and it perfectly explains Luxu's expression when the Master whispered to him whats inside and possibly why he warned him not to open it.