1. Mission 46 - Metal Gear Solid V: The Phantom Pain
Mission 46 is one of the most significant hidden levels of 2015, and it's not easy to unlock. As well as completing the previous entry on this list, Side Op 150, to unlock it, you also have to: Complete all the yellow-marked Side Ops missions in the game (including Secure Quiet). Keep upgrading Mother Base until Mission 45 becomes available to you. Complete Mission 45, which will lead to Mission 46. Mission 46 is called 'Truth - The Man Who Sold the World', in which you replay the prologue, but with one key twist that reshapes the ending of the entire game, and is the true 'canonical' ending to The Phantom Pain.
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