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Kingdom Hearts: Birth by Sleep Final Mix has a bunch of new features. The biggest is a secret episode with a new area to explore. The area, according to retailer Game Star, is the world of the secret movie from the previous title*. Square Enix created a new map and story scenes.
Other details mention the game has nine additional missions plus boss fights with characters like Monstro and an armored character.
(*Note: It isn’t clear if the previous title is the original Birth by Sleep or another Kingdom Hearts game)
