Percent Damage Attack

This is an attack or spell in many RPGs whose damage is proportional to the target's current HP; the most common forms will reduce the opponent's HP by 1/4 (or 1/2) of its current value, weakening the target without killing it.

Like a Fixed Damage Attack, these attacks ignore the game's usual damage calculations altogether, with the only variable being whether or not the attack actually hits the target to begin with.

Naturally, a character at full HP will receive the most raw damage, while characters with low HP will receive hardly any whatsoever. Whether or not this attack is capable of actually landing the killing blow varies by system: The amount of damage inflicted is typically rounded down, so a character reduced to their last Hit Point might receive no damage whatsoever (unless Scratch Damage is required, or if Elemental Rock Paper Scissors gives it a boost).

Sadly, most Percent Damage Attacks become a Useless Useful Spell in the hands of the player: Despite that enemies have more HP than the heroes (meaning the attack can inflict greater damage), enemies will frequently evade the attack or simply be resistant to them (as a rule), and and Bosses have Contractual Boss Immunity against it.

An occasional variant is a Fixed Damage Attack calculated using a percentage of the target's maximum HP, rather than their current HP.

Compare HP to One, a more extreme attack that removes all but the last Hit Point from its target.

Role-playing games

 * The Final Fantasy series has many examples and frequently classifies them as a "gravity" element, featuring versions that remove 25%, 50%, and 75% of the opponent's HP, respectively. Some can target multiple foes simultaneously, and are far more likely to succeed when used against your party members than against monsters.
 * In Final Fantasy VII, Contractual Boss Immunity is avoided for some flying bosses, who only have a 50% damage reduction from it instead. Also, there is at least one enemy that absorbs gravity attacks as HP: Master Tonberry.
 * There was also one Boss in Mook Clothing whose HP total was so high that gravity spells, while still effective, repeatedly hit the damage Cap of 9999.
 * In Final Fantasy IX and Final Fantasy XII, gravity-based damage was calculated based on an opponent's maximum HP rather than their current HP and therefore became a Fixed Damage Attack.
 * Final Fantasy X 2 has both flavors of percent-based attacks all over the place.
 * In Final Fantasy Crystal Chronicles, the "Gravity" spell could be used once per battle against a few bosses to inflict a percentage of damage. It also temporarily immobilized airborne opponents after damaging them, making them easier to defeat.
 * In Persona 4, while most Light element attacks were instant kills, one attack called "God's Judgement" reduced the target's current HP by half. Being part of the Light element, it could still be nullified/reflected by an appropriate Persona.
 * In Digital Devil Saga, most Hama/exorcism spells reduce HP by a percentage with a decent chance of working (both of those depending on the spell), but they will instant kill enemies weak to them (Mudo/death is always a One Hit Kill).
 * The "Super Fang" move in Pokémon reduces the opponent's HP by half, and is mandated to inflict Scratch Damage if the opponent is already reduced to one Hit Point (which may not be what you want in a Pokemon game). It is not affected by the game's Elemental Rock Paper Scissors beyond the fact that "Ghost"-type opponents are immune to its element.
 * The moves "Crush Grip" and "Wring Out" obey the standard damage formula (attack/defense power, Elemental Rock Paper Scissors, chances of a Critical Hit), but their base attack power is proportional to the opponent's percentage HP. If one strike reduced the opponent's HP by half, the next strike will only hit for half as much.
 * In the Pokémon Mystery Dungeon spin-offs, collateral damage caused by "Selfdestruct" and "Explosion" was always 50% of the teammate's current HP (or if they were Fire-type, 25%).
 * In Secret of Mana characters inflicted with "Petrify" status also lost 50% of their current HP as an immediate side effect.
 * The Moon Godbeast Dolan in Seiken Densetsu 3 posessed an attack that reduces a party member's current HP by half.
 * The Eclipse Dark Tome reduces the target's HP by half in Fire Emblem.
 * In addition to various HP to One attacks, Chrono Trigger features several bosses (mainly Dalton and his Golems) who use attacks that drop your health to half of its current state.
 * Runescape has enchanted ruby bolts which have a chance to take away 20% of opponent's health from the enemy at the cost of decimating player's health. They're used to take down some of the stronger enemies.
 * Golden Sun's summons use the target's max HP to calculate damage, but they also have a fixed base power.
 * Inverted in Tales of Maj Eyal: the Chronomancy spell "Echoes from the Past" deals damage equal to a percentage of the damage the target has already taken rather than how much HP it has left.
 * League of Legends has a few. Dr. Mundo's infested cleaver is like the above definition. Others include dmg calculated fsrom the targets max hp rather than his current hp.

Other genres

 * In the first Civilization, nukes destroyed exactly half of a town's current population, including defending units and one significant structure.
 * The Sorceress in Diablo II has the Static Field skill, which does 25% damage to any enemy, even a boss, though some targets cannot be dropped below a certain level this way.

Other

 * The word "decimate" originates from a Roman military punishment where soldiers were ordered to draw lots, with one out of every ten men being executed as a result. The term is rarely used that way anymore.