Phantom Doctrine

This can only be unlocked in Extended Mode. The ability to play on Extended Mode only becomes possible if you have already beaten the game at least once.

This happens after Chapter 7 in a sort of hidden mission. Once you beat the final mission of the game the credits will begin rolling. It will say to press "B" to return to the main menu. However, once pressing "B" it will actually load up a final bonus mission that takes place 8 years after the events of the game. It'd be easy to miss if you just closed the game at the credits screen.

It's a very easy mission and you'll be given a fully equipped squad for it. No special criteria, just fulfill the objectives that are listed during the mission.