An out-of-nowhere match against a mysterious opponent that becomes playable if you win.
Seen It a Million Times. A gaming trope, mostly seen in (but not limited to) Fighting Games, where, upon the completion of a task - whether the game itself tells you about it or not - you get to challenge another opponent, which makes his entrance through a dramatic, as-vague-as-possible "CHALLENGER APPROACHING" screen. Then, it's one-on-one. As if the tension wasn't enough by itself, you only have one chance. If you win, you can play as your challenger; but if you screw up... you'll have to access the match again, whether it's a difficult thing to do or not! On that note, see Double Unlock. See also Promoted to Unlockable for characters who later in a series become unlockable. Compare and contrast Mega Manning, which is the same concept applied to the Player Character's moveset. Compare Defeat Means Playable, of which this is a subtrope.You've
Examples of this trope in action: