Lifebreak

Lifebreak, also known as Revenge, ????, and Balance, is an ability that deals damage equal to the difference between the user's maximum HP and current HP.

Final Fantasy V
???? is a Blue Magic spell that does not actually tell you you are using it. It can be learned from the following enemies:
 * Azulmagia
 * Kuza Beast
 * Tyrannosaur
 * Wild Nakk
 * Behemoth
 * Gel Fish
 * King Behemoth
 * Mecha Head
 * Numb Blade
 * Tunneller

Final Fantasy VI
Strago Magus uses the Revenge Lore, known as Revenge Blast in the Advance version. It costs 31 MP to cast.

Final Fantasy VII
???? is an Enemy Skill learned from the enemies Jersey and Behemoth. It deals non-elemental damage that is equal to the caster's max HP minus his or her current HP for a cost of 3 MP. Since ???? requires the caster to be in a critical condition for highly effective damage, its usefulness is limited. It is a difficult Enemy Skill to learn at times, especially if trying to learn from Jerseys, as they MUST never be attacked in order to use the spell. If they are attacked, they will switch to their 2nd form which never uses ????, and due to a bug in their coding, will never change forms again, making it impossible to learn ???? from that specific Jersey. In addition, the spell is rarely used by both enemies, making the player have to wait a while in order for them to learn it.

Final Fantasy IX
Quina knows the Blue Magic spell Pumpkin Head, which applies the same rules to its damage equation, and costs 12 MP to use. Quina can learn this ability by eating a Python, Hedgehog Pie, Ladybug, Skeleton, Yeti, Basilisk, or Bandersnatch.

Steiner also has a similar attack, Minus Strike (サガク剣, Sagaku-ken), which costs 8 MP to use and can be learned for 35 AP through the Iron Sword, Excalibur II, or Rubber Helm.

Final Fantasy X-2
The Samurai Dressphere can learn Mirror of Equity.

Final Fantasy XII
The Time Magick spell Balance has the same function. It can be found on the Time Magick 4 license, which costs 35 license points. The spell itself can be bought from Yugri's Magicks in Rabanastre or from the Moogle merchant in Eruyt Village for 1,500 gil. The spell can also be performed using the Balance Mote item.

In the International Zodiac Job System version, Balance is a Time Magick 3 License that costs 40 LP and available for two job classes: Red Mage and Time Mage. The spell itself can only be found in a chest in the Nam-Yensa Sandsea.

Final Fantasy Tactics
Archaeodaemons can cast Lifebreak. Beowulf's Vengeance and Cloud's Blade Beam function exactly the same way.

Final Fantasy Tactics Advance
Templars can use Lifebreak. Snipers can use Doom Archer. However, Doom Archer's damage is split between HP and MP.

Final Fantasy Tactics A2: Grimoire of the Rift

 * Templars can use Lifebreak. It is now Dark-elemental.
 * Master Monks can use Withering Strike, a ranged version.
 * Trickster's Agitate and Heritor's Viola are area-of-effect versions of Lifebreak.
 * Snipers can use Marksman's Spite. Marksman's Spite damage is split between HP and MP.