Last Ditch Move

You're in the middle of a boss fight. It's taken you hours to grind your way up to a certain experience level, fight the enemies that came before the boss room, and fight the boss villain's five other incarnations. Now you're at the final incarnation, with only one hit point remaining. Not to worry, you've got just enough health potions. Or so you think.

You whittle down the boss villain's energy, using up every single healing item in your inventory. You use the biggest attack spells you can muster, and the boss finally keels over...

But not before hurling a fireball at you.

Before you can have your victory celebration, the fireball kills you. You die. Game over. And you have to go through all that crap again. From the beginning of the level.

Often done by an Action Bomb. Compare Kaizo Trap. See also Taking You with Me.

Normal Enemy Examples:

 * Final Fantasy VIII has some Mooks using an HP to One attack upon death as a plot point.
 * In Final Fantasy X, in the Bonus Dungeon and the Very Definitely Final Dungeon, there's a random encounter--not a boss, not even one in mook getup called the Behemoth King. When killed, he attacks with the Meteor spell, and is very capable of killing the entire party if the player hasn't ground much. General strategy is to sacrifice an Aeon to deal the deathblow and take the punishment, or to try and arrange it so that the deathblow is dealt as a counterattack, as counters cannot themselves be countered again.
 * Many, many enemies in Final Fantasy VI have some kind of final attack.
 * Serious Sam has beheaded bombers who explode upon dying. Also there are biomechanoids which shoot out the last projectile when falling down.
 * The MOTHER series has trees that make Smokey the Bear's advice look bad, with how much they tend to blow up. Thankfully, the rolling HP system means you can save yourself by saving the tree for last and rushing through the results.
 * In Ninja Gaiden 2 for the Xbox 360, there is a giant lava turtle that explodes when you kill it and unless you are blocking it will kill you as well.
 * Dragon Age's Abominations explode into fireballs upon death.
 * Bob-Ombs from the Mario games tend to explode when stomped, although there's usually a means of taking them out that doesn't set them off.
 * Spelunky's fire frogs explode when killed. This can get you into trouble, too - especially if one blows up part of a shop. There are also flying saucers which explode upon getting hit on impact.
 * The Spawn monsters from the original Quake are blobs that -- that's right! -- explode when killed. Combine that with their tendency to be incredibly hard to hit and get all up in your face, and you have a truly goddamned enemy to deal with.
 * Some mooks in Quake II will, just before dying, spray one last round of bullets.
 * Almost every Japanese soldier in Metal Slug 3 lighst a stick of dynamite just before dying and then blows up.
 * Doom 2's Pain Elementals release a handful of Lost Souls when they die.
 * Too many normal enemies and bosses in World of Warcraft to count. One of the earliest and nastiest were the bat riders in Zul'Gurub, who at low health gave you about 2 seconds warning before exploding for enough damage to kill anyone but a well-equipped (for the time) tank.
 * VOLTORB used SELFDESTRUCT!
 * Debuting in Diamond and Pearl, Stunky/Skuntank and Drifloon/Drifblim can have the Aftermath ability that damages enemies upon KO. (One's a skunk and the other's a balloon; the idea is probably is that both are filled with volatile gasses and when they lose consciousness...)
 * They explode?
 * Grunts in Halo 3 go kamikaze with plasma grenades when desperate (eg if their leader is killed).
 * Inverted in Fallout 3 with Broken Steel. Nuclear Anomaly, A level 30 perk, causes your character to release a nuclear explosion if you're reduced to 20 HP or less.
 * Abominations in Mass Effect 2 will explode upon death. Since they usually come grouped with husks, killing them at range can actually be a way of damaging all enemies around them.
 * Electro armour enemies in the remake of Syndicate release an AOE electric attack when killed. One achievement requires you to use it to kill at least three other enemies. Reactive armours explode when killed, with another achievement requiring you to use it to kill at least one other enemy.

Boss Examples:

 * It's a staple of Final Fantasy games that there will be at least one boss that casts Ultima (usually the strongest spell in the game) as it dies. In Final Fantasy IX it's actually a minor plot point.
 * Somewhat humourously averted in Final Fantasy V - There is one boss that attempts to cast Holy when it dies. It fails to do so because it has no MP to cast the spell with (unless the player uses an ether on it, of course...)
 * On the other hand, the boss immediately after that one (they are optional, but it's either fight both or fight neither - it's a sidequest in the final third of the game) will cast Flare upon death. (This can also be a humourous aversion because generally players are going to have Reflect up due to the fact the boss only uses magic - Flare can be reflected. Of course, it's not as humourous as this boss doesn't say anything, unlike the one which casts Holy.)
 * In Final Fantasy VII, the "Final Attack" Materia gives the PCs this ability, activating whichever Materia is grouped with it. Leveling up the Materia lets it be used more than once per battle. And yes, this works with anything that revives the now-dead character, Phoenix being obviously the best choice.
 * Many boss and miniboss monsters in the Diablo series have AoE elemental explosions that occur on death, which can be very nasty for Hard Core players.
 * Diablo 2 has the random monster attributes Fire Enchanted and Cold Enchanted not only giving their wielders elemental damage of the respective type but upon death they pull off this trope: Fire Enchanted monsters cast Corpse Explosion on their own corpse while Cold Enchanted monsters fire off a Frost Nova. For low-level characters, the latter is VERY nasty as cold damage slows and such monsters rarely go solo.
 * In Punch-Out!! for the Wii, Aran Ryan has one of these. In the first round of Career Mode, every time you knock him down he takes a swing at you which doesn't hit. However, in Title Defense mode he has a boxing glove on a rope that he uses like a flail. When you knock him down, he brings out the glove on a rope and tries to whack you one more time before falling down. Of course, it's not really capable of turning the tables, because he can't knock you down with it, and if you time a Star Punch right you'll instantly knock him out.
 * Pretty much every boss in Castlevania: Rondo of Blood throws out one last attack before they drop.
 * Thankfully, this last attack can't kill you (if it would, you just end up with a single sliver of life instead, but it can wreck your life bonus (and, if you hadn't taken any damage up until then, your hopes of being rewarded with an extra life for [what would have been] a Flawless Victory.)
 * Happens in Razing Storm when you defeat the Spider Tank boss via a Kill Sat. It drops to the ground (you're on a high platform connecting two skyscrapers) and fires out a TON of missiles in a bid to destroy the platform you and your squad are on, before exploding to bits. If you don't destroy enough of the missiles in time, you get a Nonstandard Game Over Downer Ending where your platform is destroyed and your entire squad falls to their death.
 * Splatterhouse bosses like this. In the first game, the Boreworm fight sees you fighting a small army of boreworms - and once the boss music stops, . The poltergeist boss, once defeated, ends the fight by.
 * Played straight to the hilt with White, the final boss of the Playstation port of Real Bout Fatal Fury Special. At the end of the second round won against him, he shoots one last fireball out of his cane. Failure to dodge or low-block this fireball causes an instant KO, whereupon White gets up and wins the round.
 * Paper Mario Huffnpuff will unleash a non-charged attack that deals at least nine damage plus one for every tuff puff he has floating around him (which one appears for every one or two hit points he loses) for a max of over fifteen when he he gets low on health. This is in a game in which the health max for Mario is sixty and that's only if he has maxed out his health with leveling up and has both of the HP plus badges equipped.
 * Egg Genesis boss in Sonic the Hedgehog 2006 tries to crash into you after its health is depleted. It isn't a One Hit KO however, rendering it somewhat ineffective if you have rings.
 * By the time you've damaged the Egg Viper in Sonic Adventure beyond repair, Sonic is left on up to four suspended platforms above a large abyss. But in the words of Tikal, "Watch out! He's up to something!" The Egg Viper's last move is to home in on you and destroy one of the platforms. Jump out of the way, quick!
 * Sonic the Hedgehog 4's Final Boss does this..
 * The final bosses of Sonic Rush and Sonic Rush Adventure do it as well. The Egg Titan tries to crush the platform Sonic or Blaze is on by ramming into it three times. After the third time, you must jump onto its weak point before falling to your doom. The Ghost Titan fires two One Hit KO laser beams to the edges of Sonic/Blaze's platform and slowly aims them at the center of the stage as the robot collapses, and you must jump into its weak point when it becomes low enough to hit. Both of these attacks are pathetic, and are probably just there to add tension.
 * Crocomire will attempt to attack you as a skeleton in Super Metroid, but instead collapses and open the way to new rooms.
 * Done very, very annoying with the Mecha Drago in Mother 3. It's a Wake Up Call Boss to begin with, but many, many people found defeat based solely on his "tumbling forward" attack, which happens right after you defeat him. To make matters worse, in past games, if a character fainted during the defeat animation, they would stay alive with 1 HP. Not in Mother 3.
 * Runescape has Nex which uses retribution prayer upon defeat, damaging everyone nearby. There are a few other bosses too.