Souleater

Souleater, also known as Dark Wave, Darkness or Darkside, is a self-damaging ability. The character sacrifices a small amount of HP and delivers a strong attack on all enemies.

Final Fantasy III
Souleater is the job command of the DS exclusive Dark Knight. It damages all enemies at the cost of 10% of the user's max HP.

Final Fantasy IV
Dark Wave is used by Dark Knight Cecil. It hits all enemies at the cost of his HP. It did not appear as a command in the North American SNES version of the game. However, during Cecil's encounter with his Dark Knight copy during his Paladin trial, the copy still uses this move exclusively.

In the Nintendo DS version, Dark Knight Cecil still has Darkness. However, it now has a different function. When used, the character enters the Darkness status for three turns. While in this state, the character's attack power doubles, all their attacks are of the Darkness element, but they will lose 10% of their max HP for a successful hit..

Any character can equip Darkness as an ability, provided you have stolen it off Odin.

Crisis Core -Final Fantasy VII-
Zack can learn Darkness by equipping the appropiate materia. By sacrificing some of his HP, the player may damage all enemies.

Final Fantasy VIII
The Guardian Force Diablos learns the Darkside Command Ability.

Final Fantasy IX
Steiner can use Darkside. It deals shadow-Elemental damage.

Final Fantasy X-2
Available to Dark Knights as a job command called Darkness.

Final Fantasy XI
The Dark Knight job class can use Soul Eater.

Final Fantasy XII
The Souleater Technick is purchasable for 35 LP in the License Board.

Final Fantasy XII: Revenant Wings
Darkness is learned by Ba'Gamnan at level 24 and consumes 25% of his HP.

Final Fantasy Tactics: The War of the Lions
Dark Knights have Darkness as a job command. Its techniques vary from the traditional usage of inflicting massive damage at the cost of the user's HP to draining abilities which saps a foe's HP and MP to replenish one's own. The latter allows the unit to sustain its assault with the former.

Final Fantasy Tactics A2: Grimoire of the Rift
The Lanista class, exclusive to Seeq, have Souleater as an ability. It consumes 10% of the user's max HP to inflict Dark-elemental damage.