Mortal Kombat X has a nostalgic secret fight for players who follow the series since its first chapter. This particular battle can be found at Viva Tower mode or Living Towers in English, and guarantee you the Trophy / Achievement "Keep it Secret". Here's how to find it.
Step 1. Go to the way Living Tower and look in the Premier Tower by a tower called Mortal Kombat 1, referring to the first classic game.
Note: If the tower is not available you will need to try again at another time.
Step 2. Face all the tower opponents normally until the sixth fight.
Step 3. In the sixth fight win at least 1 Round with a Flawless Victory, that is, without having received any blow and lose energy.
Step 4. Finish the sixth fight with a Fatality or Brutality.
Step 5. A special fight will come against Reptile and you will receive the Trophy / Achievement "Keep it Secret". This fight is a reference to a secret battle against Reptile there was in the original game. It is noteworthy that the difficulty is quite high.