Souleater

Souleater, also known as Soul Eater, Dark Wave, Darkness or Darkside, is a self-damaging ability. The character sacrifices a small amount of HP and delivers a strong dark-elemental attack on all enemies. It is usually associated with the Dark Knight job.

Final Fantasy III
Souleater is the job command of the Nintendo DS exclusive Dark Knight. It damages all enemies at the cost of 10% of the user's max HP. This is part of the reason (the other being their weaponry) that Dark Knights are good against enemies which multiply.

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, and all of the character's 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 the player had stolen the augment off Odin.

Final Fantasy IV: The After Years
Known as Darkness, it is an enemy ability used by the Dark Knight in the Final Tale, located in the Depths.

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 within a certain radius.

Final Fantasy VIII
The Guardian Force Diablos learns the Darkside command ability. It does increased damage in exchange for some HP loss.

Final Fantasy IX
Steiner can use Darkside. It deals Shadow-elemental damage 1.4 times as strong as a normal attack, in exchange for 12.5 % of Steiner's HP. Darkside will ignore Mini status if Steiner is in Mini, and also ignore Protect status when attacking.

Final Fantasy X-2
Available to Dark Knights as a job command called Darkness. A glitch exists that if the user were to equip the Ragnarok accessory, or make it so that spells cost 0 MP, Darkness will take no HP to use as well.

Final Fantasy XI
The Dark Knight job class can use Soul Eater. It adds 10% of your HP to your next physical attack, making multi-hit weapon skills and certain weapons very formidable when used with Souleater.

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

Final Fantasy XII: Revenant Wings
Souleater 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. Unfortunately, unless the enemy you are using it on is weak to the dark element, it is usually either of equal strength, or weaker then a standard attack.

Final Fantasy Crystal Chronicles: My Life as a King
Shadow Blade is a level 20 Sword Skill, which caps at level 100. It allows the user to deal additional damage at the cost of sacrificing HP, and can only be used once per day.

Dissidia Final Fantasy
Soul Eater is an HP attack available to Cecil and Bartz. When performed, Cecil or Bartz stabs forward with their sword, then slam an orb of dark energy into the opponent. Cecil's Brave attack "Dark Cannon" resembles the Dark Wave attack from Final Fantasy IV, in which Cecil charges and fires a swarm of dark energy orbs forward that swirl back towards him, knocking opponents closer.