The Black Box (黒い箱 Kuroi Hako) is a mysterious item referenced in Kingdom Hearts χ Back Cover and Kingdom Hearts III.
Design
The box contains thirteen locks — three on its left side, three on its right side, and seven on the front with the lock in the middle being slightly larger. The crest on the box lid has thirteen spikes on the outside and seven on the inside. The red sigil of the emblem contains letters which read "χsuper" in it.
Story
Kingdom Hearts χ Back Cover
Luxu is tasked by the Master of Masters to keep the contents of the Black Box a secret from the other apprentices and deliver it to an undisclosed location.
Luxu later passes through the Keyblade Graveyard, lugging the Box behind him.
Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-
While observing the fallout of the Keyblade War with the No Name, the Black Box can be seen by Luxu's side.
Kingdom Hearts III
Maleficent travels to various worlds along with her subordinate, Pete, searching for the black box, which, according to her, contains the Book of Prophecies. Luxord, a member of the real Organization XIII, has been tasked to find it, but he admits that he does not know what the box actually looks like or its contents. This leaves Sora confused on why the Organization would be searching for it. Luxord comments before leaving that the box contains "hope", further mystifying Sora.