Like previous versions of Mortal Kombat titles, Mortal Kombat X contains a hidden battle which can only be unlocked by achieving a Flawless Victory in a particular fight. *Spoilers below, obviously*
According to Youtuber JurrasicRabbit, once the player has accessed the Mortal Kombat 1 Living Tower, upon reaching the sixth battle, the player will need to win one Flawless Victory and kill the opponent with a Brutality or Fatality.
Should the player take damage, just lose and retry until victorious.
The hidden battle unlocked feature a standard Reptile as he appears in the core game.
Something fun to do at any rate.