The Black Box is a recurring optional boss that has appeared in the series since Wild Arms 3. This monster is unique in that, in order to fight it, one has to have opened all of the other treasure chests in the game, including chests that require a Duplicator to open. These treasure boxes will speak to the player and usually lets them know how many chests they have opened, and how many are in the game.
Wild Arms 3
The Black Box can be found in Chapter 2 of Wild Arms 3, in a room in Fortune Gear that requires a Duplicator to access. The quota of chests to be opened here is 320, with the Black Box itself being the 320th box.
Wild Arms 4
The quota for treasure chests in Wild Arms 4 is 178, with the Black Box being the 178th chest and found in the Desolate Railway dungeon.
Wild Arms 5
In Wild Arms 5, there is a Black Box near the Twelbit area with a quota of 362 chests to be opened, with the Black Box being the 362nd. In addition to that, the Wild Arms series' tradition of alluding to past terminologies is in full effect for the 10th anniversary game, as the final dungeon of Wild Arms 5 also happens to be called Black Box. However, since the rest of the world map cannot be accessed once the player has reached this dungeon, there are no treasure chests in this location that count towards the quota of chests that need to be opened in order to fight the optional boss.