Upon completing the game on normal mode after the credits you will be informed that you can press LRLRLRLRLR on the title menu to access something secret. This is easy an easy to miss prompt. The code gives you access to the Dark World and unlocks The Cool Army Code trophy. Inputting this code before completing normal mode will give you the trophy but not give you access to the Dark World. You'll know the code has been input correctly after beating the main campaign if the sky turns dark. The Dark World is the exact same game stages but with vastly increased difficulty and 100% capture rate of heroes.
