There's one Trophy/Achievement that's somewhat difficult to achieve in Mortal Kombat X, with the unlocked award entitled "Keep it Secret".
It seems it refers to a secret battle players must unlock in the Living Towers, and someone's figured out how to unlock it, sharing the solution on their Youtube channel.
This particular secret fight is connected to a tower that recreates the original tournament from the first Mortal Kombat. If all requirements are met, a secret fight against Reptile will unlock (and so will the Trophy / Achievement), who was a secret opponent in the original game.
To confront Reptile, you need to win the sixth combat in that Living Tower with a Flawless Victory and perform a Fatality.