Mirage Arena

The Mirage Arena is a new world introduced in Kingdom Hearts Birth by Sleep, specifically for the game's Multiplayer Mode, Joint Struggle. Only appearing in this mode, no story information exists for this world.

Joint Struggle
In the Mirage Arena, a gameplay mode for up to three players known as Joint Struggle is held. Here, players can interact in the Lobby and later battle the Unversed as a team. Joint Struggle features unique abilities only available through it, and allows bonuses to be obtained for Story Mode.

Setting
From outside, the Mirage Arena appears similar to Space Paranoids. A central orb in the middle, several protrusions and wrap-arounds surround it.

From inside, however, the world has an appearance similar to that of a space station. In the Lobby, glowing magenta steps highlight a vast gray floor. A wide window gives a view of space, a Moogle stands in the corner running the Medal Shop. When you set records in the Arena's games, you can get medals and exchange them for rare items in the shop. Blue data screens are also displayed on the window. In the Battle Arena, red screens surround the players as they battle Unversed on a terrain colored varying shades of blue. The surrounding space can also be seen through the windows in this area. A third area (another battle area) places players in a room with a dark floor and walls.

Trivia

 * Based on the name "Mirage Arena", it is suggested that the Mirage Arena is not a true world at all. This is supported by the fact that the world does not appear in the main story mode, does not appear in any other game in the series, does not have a Battle Level, and does not contain any treasure. This leads to the confirmed conclusion that the Mirage Arena was merely created to host Joint Struggle.
 * Its name could also refer to the fact that more than two of the same character can be used in multiplayer since the word mirage comes from the French word, "mirror", and seeing reflections or doubles is a common occurrence of a mirage.
 * Players have the ability to change the colors tones of their armor, allowing them to give the characters, Terra for example, the darker armor they had in the secret ending.