Black box

The Black box is a large chest that the Master of Masters left Luxu before disappearing in Kingdom Hearts χ Back Cover. Its contents remain unknown.

Kingdom Hearts χ Back Cover
Before disappearing, the Master of Masters left his apprentice Luxu in charge of both his Keyblade, No Name, and the black box. He made Luxu promise to simply keep it with him out of sight, and to never open it. However, to satiate Luxu's curiosity, the Master of Masters tells him what is inside.

Luxu, when he disappears, takes the box with him.

Kingdom Hearts III
Maleficent at some point became aware of the box and seeks it out with Pete in the time leading up to Master Xehanort's Keyblade War efforts. However, neither of the two know what is in the box or what it looks like.