Fatalities. It's why we started playing Mortal Kombat in the first place, and you shouldn't be surprised to hear that Mortal Kombat X not only has a ton of them, but likely the most brutal finishing moves of the series thus far.
Here's a list of all the known Fatalities in the game. Choose the character, win the match, and make sure you're the appropriate distance from your stunned opponent before you start entering the moves. Close should be right on top of them, Mid should be about a sweep length away, and far can be on the other side of the screen.
We'd say good luck but with this knowledge the only people that will be needing luck will be anyone that tries to fight you. Enjoy.
Cassie Cage
Bubble Head (Mid) - Forward, Down, Back, Forward, Square
Selfie (Close) - Down, Forward, Down, Back, Circle
D'Vorah
Bug Me (Mid) - Back, Forward, Back, X
Heart Broken (Close) - Forward, Back, Forward, Triangle
Ermac
Inner Workings (Mid) - Down, Up, Back, Square
Head Out (Mid) - Forward, Down, Down, Up
Erron Black
Sandstorm (Mid) - Down, Back, Forward, Down, Square
Six-Shooter (Far) - Back, Forward, Back, Forward, Triangle
Ferra / Torr
Better Than One (Close) - Forward, Back, Forward, Back, X
Play Time (Mid) - Down, Back, Forward, Back, Back
Reptile
Bad Breath (Far) - Down, Forward, Down, Forward, Circle
Acid Bath (Close) - Down, Down, Back, Forward, Square
Scorpion
Stop Ahead (Mid) - Down, Back, Forward, Triangle
Who's Next? (Mid) - Down, Back, Forward, Up
Shinnok
The Grinder (Mid) - Down, Back, Forward, Down, Up
Flick Trick (Mid) - Down, Up+R2, Down, Up+R2
Sonya Blade
Target Marked (Close) - Down, Down, Back, Forward, R2
Head Hunter (Close) - Forward, Back, Down, Back, Triangle
Sub-Zero
Chest Kold (Close) - Back, Forward, Down, Back, Circle
Bed of Ice (Close) - Dadn, Back, Down, Forward, Circle
Takeda
Whip It Good (Mid) - Down, Forward, Down, Back, Triangle
Head Cage (Mid) - Forward, Back, Down, Down, Square
