A Mysterious Black Box is an item given to Luxu by the Master of Masters the contents of the box are currently unknown.
Description
The Box is rectangular shaped with 20 locks and has a large red symbol on the front with the words X-Super on it.
Story
Kingdom Hearts χ Back Cover
The Master also gives Luxu a black box and explains that its contents must remain a secret, as well as the fact that Luxu is never allowed to open it. However, with Luxu's brimming curiosity, the Master reveals its contents to him, which shocks Luxu and makes him question the Master's reasoning for placing what he had inside the box. The Master merely teases that Luxu will have to wait and see. Luxu then sets out to fulfill his role, disappearing as he travels through the Keyblade Graveyard with the mysterious box and No Name in hand.
Kingdom Hearts III
In the Kingdom Hearts III Orchestra Trailer Maleficent and Pete are searching for the black box.