The “fatality” move was a selling point of the (very bloodletting) Mortal Kombat series. Unlike any old one-hit kill move, a fatality is strictly reserved for a brutal finishing move.
After performing what would usually be a final round K.O., a fatality is a sort of special move performed using a command that you can only use at that time. These commands are character-specific, and sometimes difficult to perform (though usually the problem is memorizing the command).
As one might suspect, this move is fatal, and often very bloody.
It won’t work on/with certain characters, for definite reasons (e.g., no cutting up Nakoruru or Rimururu).
Halfway between a fatality and a one-hit kill, the opponent is allowed to move.
Essentially, the opponent must have already lost a round; when they are low on life (such that the lifebar turns blue to indicate that they may enter Mu no Kyōchi), one may use a Rage Explosion, and input + CD.
If the ensuing motion hits, grim taiko-and-flute music will start playing, and the opponent will be destroyed in whichever manner one’s character uses.