Kill (XXXX on the NES and Doom in Final Fantasy Origins) is a level 8 Black Magic spell which inflicts instant death to a single enemy. This spell will always succeed if the target has less than 300 HP and is not resistant to the Death element. Otherwise, it will always miss.
The spell can be bought at Gaia and can only be learned by the Black Wizard class. In the GBA/PSP/iOS releases it costs 40 MP to cast.
Kill is a level 6 Black Magic spell. It slays all targets ten or more levels lower than the player. Note that the player will not earn experience, JP, or pg. This will not work on certain enemies, such as bosses. It costs 16 MP. It can be bought Grandship (ch 5) for 6400 pg.