The Ghaj is an enemy that appears in Chrono Trigger, in the Black Omen. Attacking this enemy with a physical attack will cause it to counter by instantly killing the attacker.
Ghajs are noteworthy for being one of the few foes capable of having Magic Capsules Charmed from them. Since they do not respawn, it is highly recommended that the player have Ayla charm all of the Ghajs in the game.
This enemy, strange though it may be, seemingly doesn't attack any of your party members by itself, only counterattacks your party members' attack(s).
A similar foe known as the Red Ghaj exists in one of the Dimensional Vortexes; unlike their normal kin, they do respawn, although Red Ghajs do also have the ability to have capsules Charmed from them.