This trope is for any of those attacks that appear to do a ridiculous amount of damage, but on closer inspection actually have the rather strange effect of reducing one (or more) party members down to their very last Hit Point instead of inflicting normal (possibly lethal) damage. Lesser versions may not leave the target with strictly 1 HP, but relative to the scale of damage typically dealt in these battles, the end result is the same — regardless of your character's HP beforehand, you are now on the threshold of Critical Existence Failure.
So what happens if you already have only one Hit Point when struck by this attack? In most cases, that amazing psychedelic rainbow shockwave (or whatever) attack will have no effect whatsoever, or may be labelled an automatic "Miss". However, sometimes even this attack is obligated to inflict a minimum of Scratch Damage and will actually kill the character unless they heal themselves (even slightly) in the meantime.
Should you fall victim to these kinds of attacks, your safest action is simply to Heal Thyself the first chance you get ... or you can also use this chance to unleash a Desperation Attack of maximum power. But be very wary: Even Scratch Damage leads directly to a Critical Existence Failure.
Related to Slap-on-the-Wrist Nuke. Poisonstatus effects often have this limitation.
In multiplayer games, suffering this can leave you open to Cherry Tapping.
Compare and contrast Last Chance Hit Point for cases where the ability to survive an attack with 1 HP is associated with the defender, not the attack itself.
Beatrix has this as her Finishing Move in Final Fantasy IX. When she executes it, the battle is over; from a story standpoint, the party is defeated as if it's a KO. She considers this her ultimate move and taunts/threatens the party with death after she uses it, which is very strange considering you can regen out of the critical status while the game fades to black, reducing the "impact" of the scene.
The final boss, Necron, boasts his "Blue Shockwave" attack, which cuts the victim down to 1 HP.
The final monster Zidane has to fight during the "You're Not Alone!" sequence has an attack that does this, which trigger's Garnet/Dagger's entrance by casting Curaga on you (even if they haven't learnt it yet).
The Blue magicDeath Claw allows Blue Mages to reduce monsters (and a few midbosses!) to single-digit hp with a successful casting. It stuns them too, leaving them unable to heal in preparation for the follow up.
Final Fantasy IV's "Tornado" spell (and its upgraded counterpart, "Maelstrom", usable only by certain enemies) reduces its target(s)' HP to single digits, providing a much the same effect.
Final Fantasy IV: The After Years loves to do this during boss fights. Adamantoise in Yang's Story is one. It also happens at the end of The Lunarian's Story too, after the fight between Golbez/Fusoya and Zeromus's Malice.
Final Fantasy VI: The spell "Fallen One" ("Heartless Angel" in the GBA translation) makes all of your warriors' HP fall to 1. When you meet Kefka at the very end of the game, this is his opening move. Other enemies besides Kefka can use it, but it usually involves using Relm's "Sketch"/"Control" command or one of Gau's rages.
Holy Dragon 2 in the GBA edition of Final Fantasy VI can randomly Counter Attack with Heartless Angel, so if your timing is off, he could counter with it and immediately follow up with his normal turn consisting of a spell that hits the entire field.
Final Fantasy VII: Sephiroth can use "Heartless Angel" in his One-Winged Angel form at the end of the game. He even kept it for his Bonus Boss fights in Kingdom Hearts (in which it also reduced your MP to 0, meaning you had to recover from it with items) and his playable appearance in Dissidia: Final Fantasy (which mercifully only takes your Brave Points down to 1), so in a way, it's almost become his trademark attack. It's also used by him in Crisis Core during the Nibelheim event.
Ultimecia's penultimate "Hell's Judgement" attack in Final Fantasy VIII causes the entire party's health to drop to one.
During the second Laguna dream sequence in Final Fantasy VIII, the last Esthar Soldier (Terminator) has a final attack called "Soul Crush" that will put Kiros and Ward down to 1 HP. Kiros and Ward are severely wounded in the following cutscene after the battle
Final Fantasy X-2 has Yojimbo and the Magus Sisters, the former being much more annoying as it also knocks the party's MP to one; you're pretty much screwed without an Alchemist
The game has special event enemies that can bring down the players' HP to 1. It's not as horrible as you'd think, considering everyone gets capped to level one for the event.
Tonberries have an attack called "Throat Stab" that brings you down to 5% HP. Luckily, it has a very short range, so you can avoid it by walking away when he tries to use it. And if you do get hit with it, it resets your accumulated enmity, so unless you're alone the tonberry will attack someone else (most likely the mages healing you to full).
Certain Diremites also have Tarsal Slam, which is HP to 1, but like Throat Stab also comes with a hate reset.
Final Fantasy XIII has Orphan who uses an attack called Merciless Judgement to bring your party's HP to 100 (relatively speaking, a critical amount). Easily fixed by using a paradigm with healers, but an instant kill if anyone is poisoned when he uses it.
The final boss of Lightning Returns: Final Fantasy XIII also uses the the Heartless Angel, but it's made much more manageable by the fact that you can see it coming in advance and guard it to prevent it from working.
Several Final Fantasy games had Cactuars with two attacks: One would do 1 HP of damage, the other would do <Player's current HP - 1> damage.
In Dissidia: Final Fantasy, hitting an enemy against the wall does bonus damage via "Wall Rush", but if the damage from the 'bonus' attack would KO the opponent (e.g., opponent has 1236 HP, damage dealt is 930 (actual attack) + 421 (Wall Rush) it brings them down to 1 instead.
Which can also be used by various mooks, such as Gargoyles on the Mountain of Woe.
The Nus in Enhasa either do nothing, reduce you to 1, or reduce you by 1. They are nice enough to attack in turn, hitting (at worst) all three characters to reduce them to one, giving you time to heal, and then hitting you with the 1 HP damage.
Krawlie, a boss that appears in the Abandoned Sewers in 2300 AD, may reduce a single character's HP to 1 with his Mash attack.
Mother 3 has the New Years Eve Bomb, although it can only be used on one boss. Said boss has so much HP that you need it anyway.
Mario & Luigi: Superstar Saga does this with the Final Boss. Both characters are reduced to 1 HP in an unavoidable attack, just in time for the boss to attack about four times (with new attacks you haven't seen before unless you've already died here) before you even get a chance to heal (barring extreme Level Grinding or use of equipment with the First Strike effect). You can survive, however, by correctly timing Action Commands to dodge the attacks.
The final boss of Metroid Fusion does this the first time she hits you.
The titular Metroid in Super Metroid reduces Samus' energy to exactly 1 point, regardless of any health tanks she has. This is more of a plot point than any serious danger.
Also in Super Metroid, the final boss (Mother Brain) resorts to the Laser Brain Attack when dealt enough damage, draining three Energy Tanks (or six, if the player is not wearing the Varia Suit), about sixty-five Missiles, and Samus's entire supply of Super Missiles and Power Bombs. What qualifies it for this trope (given that the player can have up to nineteen Energy Tanks' worth of health by the end of the game) is that not only can it not be dodged, but she'll keep doing it until the player's health becomes low enough that one more Laser Brain Attack would be lethal; furthermore, if the player has more than one Energy Tank remaining, she'll rub in her imminent victory by launching weaker attacks at Samus (who at this point is too exhausted to move) until this is no longer the case. Finally, she'll attempt to finish Samus off with one final Laser Brain Attack - at which point the infant "Super Metroid" will rush in and save Samus's life.
In the Half-Life series, Poison Headcrabs can never kill the player, but only reduce their HP to 1 (which slowly regenerates to a few below the initial value). Because they tend to be located in chaotic scenes with lots of enemies that can kill the player, people would drop everything else and unload entire clips of ammo just to kill the tiny, scuttling, black thing somewhere in the room. Even the sound effect by itself, outside of the game setting, can make most former players jump a mile.
Cutscene attacks in Super Robot Wars do either 150%, 110%, 95%, 90%, 50% or (in one event in Alpha 3) 500% of the target's maximum HP. When it's the second or third on an enemy, it generally means that the boss is either going to make a dying speech or Motive Rant before blowing up anyway, or it's going to regenerate all its health so you can fight it for real. That one time where it was 500%? It involved a Goldion Crusher on Palparepa.
And then there's the "Mercy" spirit command, which, if it works, makes any attack deal just enough damage to reduce the target's HP to 10 without killing them. It's useful for keeping high-level enemies alive so weaker characters can kill them for experience. It's also used as a minor plot point in Super Robot Wars Z, where Kira Yamato in his Thou Shalt Not Kill phase always has the Mercy effect.
In the Endless Frontier spinoff, Mercy is instead turned into a negative status effect that can only be gotten via one of the two Spirit commands that cast a random SC on either themselves or a random party member: the reason it's a negative status effect despite doing the exact same thing is because the whole point of the game is overkilling every enemy in overly elaborate ways and the only HP you ever need to worry about hitting 0 is your own.
Similarily, Heero Yuy's "I'll Kill You" command in the SD G Generation games for the Nintendo DS increases his attack power at the cost of, ironically, not being able to kill.
In Mega Man Battle Network 3, when you first fight Bass, his aura is impenetrable, and his attacks are impossible to avoid fully. He 'deletes' you, but you go back the out-of-battle screen unconscious and with 1 HP.
Also invoked with the Undershirt chip and program (or wave ability in Mega Man Star Force), which lets you retain 1 HP on an otherwise fatal hit.
Banjo-Tooie has the Stomponadon and the crushers in Grunty Industries; being crushed twice in a row results in death. Banjo's Snooze Pack move allows him to recover health in between repeatedly getting flattened to one honeycomb. One of Weldar's attacks, the ground pound, has the same effect as well.
In Secret of Mana, some chests would contain a "Death Trap". If you had full health, it would reduce your HP to one. If you didn't, instant death ensued. You very quickly learned to heal up before opening chests.
Mabadi, a level 6 Priest Spell in Wizardry: Proving Grounds of The Mad Overlord. The spell was very nice in helping kill those Murphy's Ghosts.
World of Warcraft: Raid Boss Prince Malchezaar has Enfeeble that works this way, but it's only temporary damage. The trick is to avoid any further damage until the effect wears off. Of course, he casts an area-effect spell soon after, so the targets need to run away from him (and avoid the Infernals area damage). The effect also prevents any form of healing, so there is no easy way out. Mercifully, he won't use it on whoever is on top of his threat list. This would keep it from being a completelyLuck-Based Mission, but the above-mentioned infernals do a fine job of making it one on their own.
Baron Ashbury in the revamped Shadowfang Keep has an ability called Asphyxiate, which stuns all players and reduces their HP to 1. Immediately after casting Asphyxiate he will cast Stay of Execution, an interruptable ability that heals both the players and himself for 10% of their maximum life every time second it ticks. Defeating him without letting him heal at all awards the Pardon Denied achievement.
Chimaeron in Blackwing Descent has an interesting take on this. There's a friendly NPC that prevents any hit from killing anyone if their health is above 10,000, instead causing them to live with 1 health. This means that healers just have to keep everyone above 10,000 health for most of the fight, but this effect gets disabled periodically, and so massive healing is required to keep everyone alive through his caustic slimes.
Some of the pets in the pet-battle minigame have an attack that explicitly has a version of this effect (that is, the attack does normal damage, but it will not reduce the opponent below one hit point). This has its uses, since you capture pets by reducing them to low health without killing them (then following up with a "cage" attack). If your pet has the HP to One attack, you can use him even if he's much higher level than the opponent, and not worry about an ill-timed critical hit killing the other guy.
Fox Face from Shadow Hearts has his "!!!" attack. He's got another one that reduces you to one Sanity point. There is also the ability "Nightmare", which summons thousands of cockroaches (which crawl all over the screen, too).
The Eclipse spell from the sixth Fire Emblem game reduces the target's HP to one. In the seventh game it cuts the target's HP in half (unless it crits.) However, its base accuracy was so terribly low that it barely hit unless you were extremely unlucky.
In the tenth game, Whispers gain the mastery skill Bane, which instantly reduces enemy HP to one. Of course, considering that the chances of you getting any Whispers other than Sothe, combined with the chances of Sothe turning out to be any good, this is rather worthless.
Also your other characters will often have a mastery skill that is basically "ton of damage. If he manages to survive somehow, halve his speed for 5 turns." In fact, this is the case with most mastery skills. Most of these added effects tend to be completely irrelevant since most of the skills are overpowered and will seldom not kill everything. Of course, no generic enemies nor non-plot relevant bosses will ever have these skills (or any non-mastery skills, unless they're bosses) because they never make it to the third tier, for obvious reasons.
Bane is essentially just Lethality and Mercy in one skill. The former (only belonging to Volke) kills things instantly (unless they're counted as a boss character) and the latter (belonging to Elincia) makes it so that it's impossible to kill anyone since your character will stop doing damage after an enemy's hit one HP. Mercy has its uses, but the game puts it on Elincia after a certain point in the game, but doesn't tell you, which can cause players to accidentally head out into battle with her being unable to kill things.
Because Mercenaries 2 is ridiculously easy, all exploding vehicles, airstrikes and other 'deadly' things will always jam your HP at 3, and give you about ten seconds of invulnerability. Sometimes the game totally jams at that point, meaning your Magic Regenerative Health doesn't take it back up, and gunshots can't take it back down. Useful, if you don't mind the screen being completely red.
While the Pokémon series has no actual HP to One attacks, the Generation 2 move "False Swipe" is an ordinary attack (similar to "Scratch"), but features the added effect that it will never reduce the opponent to zero HP. It is useful primarily for weakening wild Pokémon so they can be more easily caught.
The Focus Sash held item (and the Sturdy ability, in Generation 5) allows the Pokémon holding it to turn almost any potential KO attack against it into one of these, as long as it was at full HP. It will be left at 1 HP if the attack would have otherwise caused fainting. The Focus Band held item can do the same thing no matter how much HP the Pokémon holding it has, but it has a low chance of activation.
Endeavor reduces the opponent's HP to that of the user (if the opponent has higher HP, of course). Logically, Endeavor becomes this if its user has 1 HP left, which is its most common use.
In the Pokémon Mystery Dungeon series, some moves that would normally cause the user to faint in the main games like Healing Wish and Memento instead lower the user's HP to 1.
In Yu-Gi-Oh! 5Ds, this is literally the effect of Earthbound Immortal Wiraqocha Rasca; by skipping the battle phase, your opponent's Life Points become 1. This effect was removed from the real card, because an effect like that would be ridiculously broken in the real card game.
Neverwinter Nights used the Advanced Dungeons and Dragons version of Harm. As a cleric spell it wasn't resisted by the high spell resistance that most powerful enemies had, making it one of the most useful spells in the game.
Lampshaded (like everything else) in Adventurers! as one of Boss Khrima's attacks.
Shin Megami Tensei IV also has an attack called Death's Door, which causes the opponent's HP to go to one if they have the Sick status effect. Also in the boss fight against the White masquerading as Issachar, he will give you multiple Hannibal Lectures throughout the fight and one of the consequences of falling into one of them is that Flynn's HP goes down to one and his SP to zero.
Last Scenario has several bosses who can do this: Thanatos, who does this to the entire party unless you have damage-reducing Status Buffs active, Durile, who targets a single character, and the Black King, whose attack hits the entire party, too.
Birth By Sleep Final Mix has Bonus Boss No Heart, who has an attack that is basically DHA turned Up to Eleven. It doesn't just drain your HP to one, it completely drains your focus gauge and even your command board. Thankfully, he doesn't use it often, and the attack itself is much easier to dodge than Sephiroth's.
Kingdom Hearts 3D does it again with the (this time required) boss Anti Black Coat Nightmare. He extends a line of darkness towards you, and if it catches you, it drains your HP to one. Luckily, it's easy to dodge and if you do get caught, the health it drains from you is converted into HP orbs which are scattered all over the arena. You also have plenty of time to select a cure command while you're draining (though sometimes he just waits below you and finishes you off before you can use it...). However, the boss can also pick up the HP orbs, which restore a lot of his HP.
However, Deadly actually means "you have negative HP but are still conscious"; you do have to heal all that damage to get back up to 1 and beyond. Also, you can take additional damage in Deadly status, make your Will save, and stay in Deadly status an indefinite number of times. It just gets progressively less likely.
In Team Fortress 2, the Croc-O-Style previous Item Set's bonus was listed in-game as "The user cannot be killed by headshots". What it didn't tell you is that it only prevents enough damage to stop you from dying, thus dropping your HP to One (at which point another headshot CAN kill you). Players have found it's still more efficient to kill users with headshots rather than with charged bodyshots.
In Runescape, Nomad from the quest Nomad's Requiem gets a unique version of this attack. The attack always hits your maximum Life Points minus 10. In a subversion, however, the attack hits this amount regardless of your current HP - so unless your health is at (or above, through the use of potions) max, it will kill you. Some shields and armor can reduce the damage of the attack, but at the time of release this equipment was prohibitively expensive.
In Starcraft, the Devourer's Plague attack is an example of this: it is a powerful area attack but will never kill an enemy, only reduce their hit points to a minimum of 1. Subverted if you use it on Terran buildings, however, as if these are damaged into the red they automatically continue to deteriorate and eventually explode even if no further attack is made on them.
In City of Heroes, falling cannot kill you. No matter from what height you fall, you will always have at least one hit point left. That said: you will survive the impact itself, but depending on your hit points left whatever is around you can and will off you pretty quick.
This used to be used as a offensive PVP and PVE trick by Blasters who used to have an effect that their damage went up the less HP they had. They'd drop onto a team of Villians and pop off their strongest Area attack, often one-shotting a whole villain team. Defiance no longer works this way.
Then there's The Magistrium trial, where the Big Bad hits your raid league with a nuke, bringing everyone's HP to 1. This is his opening attack, he only uses it once, and it's funny because most of his attacks can one-hit-kill most characters anyway. So in essence, a point-blank hit by a nuclear bomb is the least lethal thing that you have to worry about at this point. Yikes! (For clarification, by this point your character is an "Incarnate" who's half-way up the power ladder. You're fighting someone who's made it to the top of said ladder already)
Of all games, Tekken has this move for Tekken Tag Tournament 2. Unknown has this move where she casts a vortex, and if you're caught in that vortex, a large hand rises up and slams you into the vortex, leaving you with 1 HP note (Actually, it makes all your health red, which means it's recoverable upon tag out.) and automatically tagging in your partner. If you're really unlucky to be caught in this attack and your partner has low health, good luck trying to survive.
The Fallout: New Vegas DLC Dead Money, when played on hardcore mode, features an outdoor area with toxic gas that constantly drains your health down to 1HP. Given a dearth of healing items, this basically turns you into a One HP Wonder for the entire first half of the add-on.
Defense of the Ancients: All-Stars has Doom and Poison Nova which technically do not do this but deal so much damage over time that they might as well. To counter this, finish the battle quickly and then heal.
In Parasite Eve these attacks: the crab's attack that involves grabbing and pinching you, Eve's red tentacle in Day 5, Ultimate Being's yellow shockwave, and Maya's Energy Shot, all put you down to 1 HP.
Older editions of (Advanced) Dungeons & Dragons had the Harm spell (reverse of Heal), which would strip the victim of all but 1d4 or so hit points, no save. It did the same thing in 3rd edition where, due to the revised casting, melee attack and HP system between 2nd and 3rd edition, it turned into a massive Game Breaker. The version in 3.5 inflicts damage based on caster level, but it still cannot reduce the victim's hp below 1.
An article in Dragon magazine #36 gave AD&D stats for Conan. He could not be killed by poison: instead, he would be reduced to one Hit Point and fall unconscious.
In 3.5 edition drowning set your health to zero (not "down to" zero; if your health was below zero at the time, it would actually go up to zero). This was the basis for one of the most broken characters of all time which used this, an infinite damage loop, a spell that prevented death (at least from hit point damage) for a specific period of time, combined with a whole host of spells that added benefits per x amount of damage you took. It was named the Omniscificer. What this ignored was that by the same technicality, getting out of the water doesn't stop you from drowning.
The Eater of Dreams in Captain SNES: The Game Masta has the Living Nightmare attack, which reduces everyone's HP to one and heals himself for the damage dealt. He cruelly uses it to foist a no-win situation on Lucca.