A game mechanic where undead enemies can be quickly defeated with health items or by casting healing/revival magic. From a gameplay standpoint, this simply allows healing skills to do double duty as Turn Undead, and makes the party's dedicated healer not-so-useless when your party is asked to explore that ancient crypt at night. Logically, it's often explained or assumed that the source of healing magic (usually nature or the divine) is anathema to the undead. This particular example is one of the worst cases of Guide Dang It, since it's unintuitive to cast a healing spell on an enemy if players are unfamiliar with this trope.
For overlooked techniques that are genuinely useless except for one very specific situation, see Not Completely Useless. Compare Sliding Scale of Undead Regeneration for ways the undead can heal without, err, re-dying? Subtrope of Holy Hand Grenade, where Holy magic is explicitly used to kill and maim enemies, living or not. Also a subtrope of Outside-the-Box Tactic, which covers any weakness an enemy might have that is not immediately apparent.
In Baldur's Gate 2, a quest involves defeating a monster that is completely invulnerable, but will die when even the weakest healing spell is used on it. Thankfully, this isn't hard to find out, and the game helpfully puts a somewhat-hidden healing scroll in the same room.
Clive Barker's Undying: This is literally the only way to kill the Skeleton Monks, until you get the Scythe and are able to decapitate them.
In Final Fantasy IV, Scarmiglione and his zombie minions are hurt and killed by using healing spells and items. Revive spell and items won't kill them outright, but will score huge amounts of damage.
In the DS version the second form will counter every physical or black magic attack using a gas that inflicts silence, slow, poison, and confusion. Effectively, the only way to defeat the boss without burning through all of your items was to spam Cure spells and Hi-Potions on him.
Final Fantasy V also has the Moogle Eater boss, which is That One Boss if you try to fight it legitimately, but jobs to a Phoenix Down or the Raise spell.
You could also use a Gold Needle (which cures Petrification) to get an instant-kill on Stone enemies.
Final Fantasy V was also the first appearance of the Kill Revives Zombie inversion in the series. It made the Assassin's Dagger pretty much unusable in certain areas.
While using a Phoenix Down in Final Fantasy VI will kill any undead enemy in one hit, they can be expensive to stock up on. Holy Waters turn out to also kill undead enemies instantly, but they cost quite a bit less.
The Phantom Train drops from one Phoenix Down.
Final Fantasy VI was supposed to have the reverse to this system as well: while curative magic and items healed regular characters and hurt undead enemies, Seizure and Phantasm would hurt regular characters and heal undead. Unfortunately, due to one of the numerous bugs in the game's battle system, the status effect didn't work like it was supposed to, and actually ended up hurting the monsters that used Seizure instead of healing them. The resultant battles are rather humorous to watch.
Another "counter" to this system is the exact reverse of Revive Kills Zombie, kill revives zombie. Using Instant Death effects like the Death spell or the effect of an Assassin's Dagger will on an undead foe will cause them to die... and then instantly regenerate with full HP.
Also interesting is that the zombie damage system can be applied to player characters. Anyone wearing a Lich Ring is turned undead without the side effects of the Zombie condition, and so will be healed by Death and Poison, and harmed by Cure. Also applies to Gau raging an undead enemy.
A ghost boss in Final Fantasy VII could be killed instantly by using an X-potion (restores a living party member to full HP) on it, since it had less than 9999 HP.
Casting "Angel Whisper" (ultimate cure-everything-even-death spell) on an undead enemy will result in instant death (no HP loss) + many status ailments.
Because Final Fantasy VII's version of White Wind also cured status effects, using it on an undead enemy would instantly kill it via petrification unless the monster in question was immune.
Then there's the Zombie President in Final Fantasy VIII which transforms into a zombie after a few hits. After it transforms, it can be killed by a single Phoenix Down. (The success rate, however, is quite low, so it actually takes a relatively large number of Phoenix Downs to kill him.)
Abadon from the same game, a Phoenix Down will miss but the Curaga spell will severely damage him. For some unknown reason, he has the spell himself so you don't even need to use up your own magic stock, just keep drawing and casting!
The Zombie status effect makes player characters subject to this trope, as well as turning their models a strange shade of green. This is its only effect, so you might wonder why the enemies bother. Right up until the point you get one-shotted with a Curaga.
In Final Fantasy IX the standard cure reversal works, and Life and Full Life both kill undead monsters instantly, while Phoenix Down causes HP to One to zombies, allowing even Dagger, whose weapons are the weakest, to finish them off. Oddly enough, though, zombification doesn't wear off upon death, making the game hate you during the Iifa Tree level, where your characters keep getting zombified. You can't revive a zombified party member unless you first remove the zombie status with an item — and Remedy (the cure-all for status effects) doesn't cure zombification or viral infection.
And like Final Fantasy V, chucking a Soft at certain 'stone' enemies will kill them instantly - perfect for taking out those Epitaphs (though you don't gain XP using this method).
Evrae Altana in Final Fantasy X takes twonote a Phoenix Down in this game revives the target for half of its health. So naturally, Evrae Altana loses HP equal to half of its max to one of them. If it were not for Contractual Boss Immunity, it would be a One-Hit Kill instead.. Final boss Yu Yevon, while not a zombie, is vulnerable to zombification (unlike most boss monsters); coupled with his habit of casting a very powerful cure spell during any turn in which his life is not at maximum, this makes it fairly easy to trick him into killing himself.
One boss uses this exact tactic against your party, using a Zombie attack on one of your party members followed by Life (which kills Zombies). This can easily be used to your advantage, though: Occasionally he will aim for a party member he did not Zombify, causing nothing to happen. He might even hit a dead party member, reviving him with full HP. A later Sequential Boss Lady Yunalesca also resorts to Zombie effects in her second form which you must "suffer" before defeating her, because the first action of her third form is a global death effect which only Zombied party members will survive.
Almost the ONLY way for a reasonably leveled party to take on the Dark Flans inside Mount Gagazet is to use a zombie weapon to inflict the status on the flan, then Phoenix Down or Life it to death.
Aside from Cure, all of your White Magic takes an offensive bent when used on enemies. Life becomes an insta-kill spell, Heal hits the targeted enemy with every status affliction it would normally cure, and Exit boots them out of reality entirely.
This trope is the basis for one of the main game-breakers of Final Fantasy XII where you can spawn and then repeatedly kill Dustia, an undead rare monster far beyond your combat level, right at the start of the game. This allows a player to level up Vaan to level 40+ in an absurdly short stint and in turn raises all of your eventual allies levels through Leaked Experience.
In Final Fantasy Tactics A2 there is one Side Quest where a requester wants a Potion and Hi-Potion to heal up, but winds up hurting herself drinking the Potion because she is a zombie. Luso stops her from drinking the Hi-Potion, which would have been extremely fatal. Keep taking care of her, she gets better (and dual wielding).
Somewhat notably, undead enemies in the Tactics games aren't truly killed when their HP reaches zero — they're in the ground, but come back after a few turns. Phoenix Down and the Raise/Revive spells are some of the only ways to put zombies, ghosts, and vampires out of commission permanently, but they have a low probability of actually working unless the undead bad guy in question is pushing up daisies.
Played completely straight in Final Fantasy Tactics, while also adding practicality to the Life 2 spell; both versions of it were lethal to the undead, but the upgraded form was much more accurate. It also made battles against undead squads the perfect place to deploy a character with the Calculator skill; even if their spell parameters hits everybody on the field, that's hardly a problem if you're casting Cure 4 or Life 2.
Bravely Default plays this trope entirely straight, enemies with the Undead type will be damaged by curative spells. Raise and Phoenix Downs kill them instantly but there's a chance they won't work. Not much of a problem if you just command everyone to Brave to the maximum and have them all try to revive the enemy, especially with the Sage's Staff that allows free use of Raise. One boss later in the game is an undead dragon that will restore to full HP when killed, which only means you have to revive it twice, which you're probably already in the process of doing. Subverted with the boss of Everlast Tower, he can be damaged by healing spells but he won't be killed by revival spells.
In Bungie's Myth series, healing any undead unit will kill it.
A glitch/feature allows you to use this against the final boss of the first game, breaking the otherwise awesomely challenging ending with a one-hit-kill. The second game averts the trope with all undead boss and mini-boss units—healing them will actually heal them.
In NetHack, the Finger of Death spell, one of the most powerful spells in the game, resulting in an instant kill if the target is not undead, only serves to heal Death, while Pestilence can be healed by potions of sickness and damaged by potions of healing.
Played straight in the MMORPG Ragnarok Online, where the Resurrection spell and items that give the same effect have a chance to instantly kill any non-boss creature with the undead element, even other players (assuming they wear armour that gives them the undead element). Healing spells of every sort also damage undead enemies (and allies), the exception being the Alchemist's Potion pitcher skill.
Done and given an Evil Counterpart in Warcraft III: The Paladin spell Holy Light harms undead and heals the living, while the Death Knight spell Death Coil harms the living and heals the undead.
The MMORPG successor World of Warcraft ignores this for gameplay reasons: undead PCs can be healed and revived just like the living. Penny Arcade ponders the philosophical problems. However the spell Holy Shock was turned into a Healing Shiv, harming enemies and healing allies. When Death Knights became playable, their version of Death Coil could only heal their own summoned creatures or themselves.
In the Naxxramas raid, there is even a Death Knight able to wield the Holy Light. He is described as "a paladin in life, so strong in his faith, that even in undeath, the power of the light still heeds his call, smiting his foes in battle."
In the Warcraft tabletop RPGnote Though all RPG information that is not specifically brought up in other sources is considered Canon Discontinuity by default within the video game, it is stated in the rules that using resurrection spells on undead creatures won't work, or if the undead has been "killed", return it back to unlife. This is apparently because resurrection spells return the being's soul to their body, and the undead still have their soul, it's just bound to the undead body (making undeath their "natural" state). Light-based healing should still damage the undead since they're powered by the antithesis of the Light.
Word of God says that undead who keep their own will can use and be healed by the light, it just hurts like hell.
"Wielding the Light is a matter of having willpower or faith in one's own ability to do it. That's why there are evil paladins. For the undead, this requires such a great deal of willpower that it is exceedingly rare, especially since it is self-destructive. When undead channel the Light, it feels (to them) as if their entire bodies are being consumed in righteous fire. Forsaken healed by the Light (whether the healer is Forsaken or not) are effectively cauterized by the effect: sure, the wound is healed, but the healing effect is cripplingly painful."
In effect in MapleStory, where Undead monsters will immediately become Clerics' favourite monsters to grind on, as the Heal skill heals them, heals all nearby party members and damage any undead monsters in the area. They can just spam the Heal skill on mobs of undead monsters and the only thing they'll ever have to worry about is MP. Not a bad deal.
In a variation, the easy way to beat the boss fight against Cyanis in The Bards Tale III is to cast cure on him. He's not a zombie, though, just a good man gone mad from grief.
In the 3rd Breath of Fire game, you will occasionally run into a large group of Zombie enemies lead by a "ZombieDr". Wail on his team for a bit, and the good Doctor will use the game's most powerful full-party healing spell....at which point this trope turns it into one of the most hilarious things to ever happen in a random encounter in RPG history.
In the first dungeon right after the time skip, you encounter a Zombie Dragon boss who loves mass status effects and is incredibly annoying. Level-grind Garr to 26 beforehand and cast Kyrie, hilarity ensues.
As Neverwinter Nights uses the D&D ruleset, this works as expected. Players mystified by Vampire Priests who unexpectedly die in a flash of white energy might be amused to learn that, when seriously injured, any NPC with standard cleric AI might attempt to use healing magic on themselves, thus committing suicide as undead. (Irritatingly, the XP goes away, too.)
In the premium module Pirates of the Sword Coast, your character becomes undead. After that point healing spells and potions hurt you, and you must instead chug potions of harm to restore your HP. (Mercifully, if you decide to export your undead character into a different module, they'll retain all the undead immunities but can be healed with normal curative magic.)
Pokémon Diamond & Pearl introduced Black Sludge, an item that harms its holder. That is, unless held by a Poison type, which is healed instead.
In the game Magicka, most spells cast using the Life element heal the living and deal damage to zombies. Which explains the ability to place healing mines.
In Xenogears, a particularly notorious enemy not only gets healed by offensive spells, but also by basic physical attacks. The ensuing rage and confusion is usually enough to stop most players from discovering that using even the weaker heals on it drops it pretty quickly—though it seems obvious now, imagine you're experimenting with all the different elements, deathblows, lethal items, trying to find SOMETHING that can stop this thing...experimentation usually stops short of trying to heal your foes, especially when the monster's appearance and name don't exactly give away this trait. They also have an HP to One ability. This monster alone takes its place among the game's other puzzles that collectively make the average gamer feel like a dunce.
Minecraft has several kinds of potions with beneficial or harmful effects. For every type, you can use it on yourself, or turn it into a splash potion to throw at friends or enemies. Zombies, zombie pigmen and skeletons are healed by potions of Poison or Instant Harm, but can be damaged with potions of Regeneration and Instant Health.
Surprisingly (and somewhat frustratingly) averted in multipart browser RPG MARDEK, which has a Zombie status effect (which turns your party members into zombies who attack you) and an item, Holy Water, that cures it. Using Holy Water on a pre-existing zombie does nothing. Healing spells, however, do decent damage to undead.
In one of the more oddly hilarious yet annoying cases of this, Vehrn, an overbearingly religious paladin of YALORT, gets a skill in Chapter 3 called Lay Hands. It heals, and can cure curses and zombification. The catch? It deals light damage to a zombified party member Vehrn cures with it.
The Elder Scrolls V: Skyrim has a variation of this trope; Turn Undead spells are treated as part of the restoration school alongside healing magic, and characters who master the restoration school can choose to take the necromage perk, making all their spells more effective against the undead. Amusingly, characters who turn themselves into vampires and take necromage will find that both their offensive spells are more useful against undead, and buff effects they use on themselves will be more effective.
In Might and Magic: Heroes VI, healing spells with the light element can be used offensively against undead and demons. Since orcs in the setting are part human, part demon, they can use light magic for healing, but can be harmed if an enemy uses light magic against them.
Originated in the Dungeons & Dragons tabletop roleplaying system, from which a great many of the mechanics of fantasy RPGs, and the tropes based on them, arose.
This is explained within the rules by stating that undead creatures are powered by negative energy, while healing spells work by channeling positive energy when the two types meet, they cancel each other out, harming the undead. Likewise Inflict Wound spells use negative energy to harm the living, and thus heal undead creatures. Most Necromancy spells, which use negative energy, only heal Undead foes. An exception is "Undeath to Death", one of the very few instant-kill spells that can affect them.
The actual return from the dead spells, however, require material components worth thousands of GP (that are consumed by the casting) take several minutes (a minute being ten combat rounds) to cast, and explicitly state they do not work on undead creatures, at least not if the undead creature hasn't been re-killed already (in which case it turns the undead creature back into who it was when it was alive).
In second edition and before, however, it did work on undead creatures, either destroying them or turning them into living creatures depending on exactly what rule you looked at. The description of the mummy in first edition stated specifically that a resurrection spell turns it into a normal fighter. Raise Dead acted as Slay Living for undead. (Yeah, it makes sense) But then, 2nd. ed. had the entire concept of "reversible" spells...
According 3.5, undead are in fact turned back to normal by the spell true resurrection, which makes for some very interesting RP opportunities and new chars.
Averted in 4th Edition, wherein healing effects work the same on everybody, and the old "positive energy/negative energy" has been changed to "radiant damage/necrotic damage" . Undead are resistant to necrotic damage and vulnerable to radiant damage, but enough necrotic damage will still destroy undead, and radiant damage hurts the living too.
In the Endless Quest series published by TSR (a sort of Choose Your Own Adventure IN D&D! series), one ending for Lair of the Lich is to cast Raise Dead on said lich, turning him into a powerless old man. This doesn't work in the game at all of course, but hey, it was funny.
One of the odder monsters of the old-school D&D games was the Nilbog, a goblin that could not be killed with regular attacks and spells, as such attacks would heal him rather than hurt him. The only way to kill him was to use healing spells.
Magic: The Gathering does this a few times, only it's usually enchantment removal that kills zombies. (But since enchantment removal and damage prevention are both green and white abilities...) Animate Dead lets you do it to one of your creatures in your graveyard. Lich reduces your life total to zero, turns lifegain into cards, and life loss into sacrificing permanents. If you can't sacrifice one, you lose. Also, if it's removed from play, you lose. Nefarious Lich is identical, except that it has exiling cards in your graveyard. Finally, we have Transcendence, which has that damage heals you but ultimately kills you, and all that healing causes you to lose the game.
Anime & Manga
During the final battle in Flame of Recca, Mori Kouran, hoping to gain immortality by absorbing Yanagi, is instead destroyed when Yanagi, turned into one of Recca's flames, uses her healing powers to "heal" the bodies absorbed by Mori by sending them into the afterlife, leaving Mori to wither and die.
In the cat arc in Inuyasha, Sesshomaru defeats a monster that had sucked up the souls of four other demons using his Tenseiga sword, which cannot harm a living being, and revives the dead. The trapped souls return to their bodies, rendering the Big Bad's butt infinitely more kickable.
Tenseiga revives the dead by slaying the spirits that come to gather the dead person's soul. As it happens, this little quirk means that Tenseiga can cut ghosts and other Made Of Air entities, and is in fact the only weapon that can do so.
Apparently, this is an intrinsic property of Ripple in JoJo's Bizarre Adventure. It's Life Energy and thus antithetical to undead, especially Dio and his lackeys. Jonathan Joestar defeats Knight Braford by reversing him to a state of living, ending his undeath.
Many household items and materials that have a known healing effect are often associated to be used against evil spirits or creatures. For example, silver is a known germicide (it's toxic to germs like many heavy metals but not toxic enough to kill humans, at least not accidentally) and has been used instead of antibiotics throughout history. Werewolves are hurt by silver, as well as vampires and possibly other evil creatures.
According to some accounts, the nuckelavee (a skinless, plague-bearing centaur...or, "a very large head on two small arms") was hurt by contact with fresh water.
Vampires and evil spirits were believed to be incapable of crossing running water. It's probably based on the ancients noticing that drinking from stagnant water, which was often loaded with dangerous microbes, caused illness, while running water was considered safe for drinking.
In Night on Mispec Moor by Larry Niven, an alien plant reproduces by infecting newly killed corpses and rallying their bodies for one last lurch. On a battlefield an off-worlder is cornered by these plausible zombies. He's in deep trouble until, in desperation, he tries spritzing one with his pan-spectrum cure spray.
Averted in the Xanth series, where water from a healing spring can patch up damage dealt to undead creatures - fairly useful to the (good guy) Zombie Master, since his zombies don't heal naturally. It can't return them to true life, though, only return them to an intact corpse state.
The Hunter from the Coldfire Trilogy is a strange example. As a consequence of the Deal with the Devil that made him immortal, his very being is so twisted that healing magic would have no effect on him. Worse, if he tried to use it himself it would kill him. When Damian learns this he muses on the irony of being punished for an act of compassion.
Rappaccini's Daughter by Nathaniel Hawthorne is one of the older examples of this. The daughter of a botanist, Beatrice Rappaccini grows up in a poisonous garden and, as a result, becomes poisonous herself. A man falls in love with her from afar and, in desperation, gives her an antidote so they can try to live together. Guess what happens...
In The Black Mountains by Fred Saberhagen, Som the Dead, a local viceroy of a vicious empire, has made himself immortal and invulnerable by becoming a living dead. Any attacks against him wound the attacker. He is finally destroyed when, mistaking him for someone horribly wounded and gangrenous, Draffut throws a measure of concentrated liquid life force at him.
For a limited time, the hard copy of Brendan Atkins' online Ommatidia stories came with a personalised story based on the buyer's prompt. Story 34 of 101 was a fictionalised account of a battle against a boss character called The Weeping Boy, in which the text noted The Computer Is a Cheating Bastard with, among other things: "He's undead, but immune to reverse heals!"
In the third Way of the Tiger book, Usurper!, you are attacked by Ringwraithexpies, the only way to kill one happens to be scrolls of raise dead. Holy water and other attacks merely drive them off.
Live Action TV
Ricky Fitness saves his fellow Aquabats from grimy sludge monsters with his anti-bacterial hand gel. They later subvert the trope against a fairly clean "lint and cleaning chemicals" monster by using Crash McLarson's lucky socks.
In Noob, the main guild's healer accidentally heals enemies on a regular basis, which is quite annoying for his guildmates... except when they happen to be dealing with undead enemies.
Destroy The Godmodder: This is used occasionally, but once resulted in a week-long series of hilarity when one player flooded the field with variant of holy water (which was used twiced), most notably unholy water (which was used three times), each time the godmodder was somehow transformed into whatever would benefit from the flood. Culminating in the mass of floods canceling each other out and killing everything but the godmodder.
In Dominic Deegan, white magic (which usually has restorative and invigorating effects) is not only very effective against undead and necromancers, but potentially lethal to infernomancers. This is apparently not an inherent feature of the magic itself, but rather because demons (and, by extension, their mortal servants) are vulnerable to faith, and white magic has a strong association with holiness among Callanians. For the orcs, who assign little spiritual significance to light or darkness but hold ice to be sacred, ice-based magic is just as effective against demonic foes as white magic is for Callanians.
In 8-Bit Theater, Chaos had just about torn his way into the dimension in order to turn it into his own hellish playground. The depowered protagonists were helpless and as Chaos went One-Winged Angel it appeared all was doomed... until four healing characters showed up and purified Chaos with White Magic.
White Mage: And then we zapped enough white magic to bring down a vile god of chaotic energy. Priest: Which he was. Healer: So, that worked out.
Referenced in a combination of thisCtrl+Alt+Del strip and the one straight after it. Although in this case it may be that Ethan did not intend the tropes meaning. The fact that the arc so far has a heavy tabletop games theme running through it though seems to indicate that he would know about it.
Full Frontal Nerdity played this with regards to a Left 4 Dead-inspired campaign. Cure Disease would kill the zombies by eliminating the virus animating them while a resurrection would return them to life... at which point the zombies would tear the newly-revived character apart.
In Homestuck Jane's Trickster mode's life powers kill the undead enemies that inhabit her world. Jade suggests they would also kill Rainbow Drinkers (Troll vampires) like Kanaya. Kanaya thinks Jade doesn't really know a lot about how Rainbow Drinkers work.
Not undead per se, but in The Legend of Korra, the Dark Avatar is killed off using a form of spiritual waterbending that infuses water with light, which as far as it is known it can only calm down and/or cleanse spirits.
On Adventure Time,the Lich is hurt by Guardian blood, which has healing properties. It causes his powers to disappear and flesh to grow over his bones and seemingly "reboot" him into a living, apparently harmless, baby.