Complex sentence: If at some point in the game the player cannot catch a Shadow Pokémon; there is a second chance available, at random points throughout the game an antagonist Miror B. will appear and have a Shadow Pokémon that the player was not able to catch.
Split into simpler sentences: If at some point in the game the player cannot catch a Shadow Pokémon; there is a second chance available, at random points throughout the game an antagonist Miror. B will appear and have a Shadow Pokémon that the player was not able to catch.