Villain-Beating Artifact

Welcome to my cave, young adventurous Hero. Here, pick one of these three artifacts, because at your current level you can't defeat the villain or punch out Cthulhu. He, his Mooks, scientific experiment pet, and his three-year-old niece will defeat you in less than a minute if you do not use this item.
The first was forged of purest minerals and one of its special ingredients is virgin tears of joy. This artifact is able to take on and even kill Evil Incarnate itself.
The second is a whip that was handed down from the legendary Vampire Hunters, whose heir is lost to history. If their blood runs in your veins, this will boost your current spells and strength to be on an equal match with the Big Bad's strength, skills, and magic.
The third is a tome, contained within which is a powerful spell that can seal your foe away forever.
Which do you choose?

The Villain-Beating Artifact is a one-purpose MacGuffin that makes it possible for a functionally inferior hero to take out a vastly superior Big Bad. What differentiates it from the Infinity+1 Sword is that it's only useful against that one Big Bad -- against any other foe or in any other context it's basically no more effective than a wet noodle. And that assumes it can even be wielded or applied in situations outside of its intended purpose; some video game instances of the Villain-Beating Artifact are simply inert lumps occupying inventory space before and after their special use. However, against its intended target it is often a One-Hit Kill, turning him from an unbeatable monster into a Zero Effort Boss.

Some Villain-Beating Artifacts enforce their special singular purpose by being consumed in the process of defeating their designated target -- often blowing up from all the energy they channel, or being drained of all their power.

Subtrope of Plot Coupon That Does Something. Sister Trope to Sword of Plot Advancement. If the reason it's used only on one particular boss and no other is that it's simply ignored or forgotten after its one use, that's Forgotten Phlebotinum.

Examples of Villain-Beating Artifact include:

Comic Books

DC Comics has the Spear of Destiny, which is the most powerful weapon ever. It holds supernatural powers that allows its owner to control the minds of those with superpowers.
- It was used on Spectre, who is a spirit of God's vengeance on Earth! The Spear almost killed him and precipitated the world's end.
- Averted in "All Star Squadron" #20, the Spear of Destiny was given as the reason why the Justice League of America couldn't jump in and help Europe out during World War II. Hitler somehow got hold of this spear and would use it on them if they entered Nazi-occupied lands (this spear can even harm Superman), so the Justice League of America had a very good reason to stay away.
- For many years, Kryptonite was this for Superman. Despite being a radioactive mineral, it had no real use outside of hurting the Man of Steel, and non-Kryptonians could swallow hunks of the stuff without ill effect. This permanently changed Post-Crisis, when the common-sense notion that radiation was radiation and deadly no matter what the source took over. (It was starting to change even as early as the 1970s, as witnessed by the "Kryptonite No More!" storyline which started out with an attempt by someone to build a Kryptonite reactor.)
Marvel Comics has the Ultimate Nullifier, a devastating weapon the size of a smartphone. It scares Galactus and prevents him from devouring planets. Johnny Storm found it in Galactus's homeworld and uses it to save the Earth. The Other Wiki says that it can nullify or re-create anything, up to an entire parallel universe.

Film

Kubo and the Two Strings: The titular character and his companions must find three weapons -- the "Sword Unbreakable," "Breastplace Impenetrable," and "Helmet" -- in order to defeat the Moon King. Kubo has little to no training on how to use these weapons. So, he takes a third option and uses his shamisen to defeat the Moon King.
In Omen III, The Antichrist Damien Thorne's weakness is the Seven Daggers of Megiddo. Only one will do.
In the 2017 version of Wonder Woman, an ancient sword called the God-Killer is kept by the Amazons on Themyscira, guarded against future need. This is the weapon that can destroy Ares, who spreads death and destruction and has even killed of the rest of the Greek pantheon. As it turns out, the God-Killer is just a normal sword -- the real God-Killer is actually Princess Diana/Wonder Woman, since she's the last of Zeus's demigod offspring.

Gamebooks

In the Fighting Fantasy gamebook House of Hell, the Final Boss can only be defeated if the player finds and uses the Kris Knife, and even then only in a room with red tapestry, to simulate a battle fought in the depths of Hell itself. Of course, exactly why the cult keeps this weapon in the house at all (or why the eponymous House has a room that qualifies) is something of a mystery.

Literature

In Mistborn, this is double subverted. Ten metals used in the Magic System are known to bestow powers. Kelsier made rumors of an Eleventh Metal that has powers to defeat the Lord Ruler; when Vin tries to use it, it shows her intangible images of people who used to be. The second time Vin uses it, she realizes that the Lord Ruler is not who everyone thinks he is, and uses this knowledge to take out his Immortality Inducer. No Immortal Inertia ensues.
In The Sword of Shannara, the Warlock Lord was defeated by using the titular sword, whose powers shows the absolute truth about those that touch it. It's revealed that he's been dead for eons long before the novels began. Plus, the Warlock Lord was reanimated by a combo of collective fear and magic.
His Dark Materials subverts this. The subtle knife was claimed several times to be used as the only weapon capable of killing God, the supposedly Big Bad of this series. The knife's owner was encourage to take it to the guy opposing God, so he can win. Turns out that God was too old and senile and eventually dies of old age. In addition, not much of Big Bad material.
Gardner F. Fox's Niall of the Far Travels short story "Out of the Eons": Adonair, an alternate universe evil deity, tries to enslave humanity and conquer the world. The gods can't destroy him but manage to trap him. Niall accidentally releases Adonair at the story's beginning. The only way to get rid of Adonair is for Niall to drink a special liquid and trick Adonair into possessing his body.

Live-Action TV

Supernatural:
- Samuel Colt made a special Colt gun and bullets, which can be used to kill almost anything... except Lucifer.
- There's a special dagger which can kill demons (and the humans that they possess at the time). Regular knives and daggers don't work, it's just the host body. Its backstory has yet to appear.
- In one episode, the dragon can only be killed by a sword that's forged in the blood of another dragon.

Dean: "So, you need one sword to kill one dragon, but you got to kill one dragon to make one sword. How does this work out?"

Buffy the Vampire Slayer had several such items:
- The Troll God's Hammer that's used to whomp Glory down to size in Season 5.
- An ancient scythe that's Caleb's Achilles' Heel.
- Hope's Dagger, which was made from the first sun rays on earth. This is the only weapon to harm the First Evil.
Doctor Who: Subverted in the episode "Last of the Time Lords," which has Martha Jones spending a whole year searching for a weapon which was separated into four parts that can defeat the Master. It's revealed at the end that the Villain-Beating Artifact was just a ruse merely to distract from Martha's real objective.
Once Upon a Time twisted this trope. The only way to defeat the Big Bad is to steal his dagger, but you'll replace the Big Bad if you somehow manage to kill him. The Duke chose to control the Dark One with this dagger; Rumplestiltskin did not.
In Madan Senki Ryukendo, it's stated in the Kanon of Light that to defeat DaiMaOh, they need three Ultimate Keys.

Video Games

In The Secret of Monkey Island, Guybrush Threepwood found out that there's a Villain-Beating Artifact that can be used to defeat ghosts, especially Captain LeChuck. The pirate then went to find the ingredients (i.e. Voodoo root, etc.).
- In Monkey Island 2: LeChuck's Revenge spraying root beer doesn't work. Guybrush has to assemble a different VBA to defeat his foe.
In Grow RPG, this trope was distilled down to its molecular essence. Besides the sword and armor, you will need the special blue orb which can knock the Big Bad's life meter down to where you can go man to man. It's not possible to reach the Big Bag without it.
In Chrono Cross, you can kill the Time Devourer using force alone, but it'll come back eventually. The only way to defeat it once and for all is to use the Chrono Cross.
In God of War, whatever is contained within Pandora's Box is the only thing that has the power to kill a god. Kratos spends the first game to find and open the box, that way he can kill Ares. In God of War 3, he needs it again to defeat Zeus. He was lied to, since the box was empty the last time he opened it.
Warcraft 3: Not a specific weapon, but mainly a damage type. Divine armor's weakness is Chaos, which is reserved for Hell's legions or insanely strong monsters. In order to defeat Cenarius, an invulnerable demigod, Grom Hellscream's orcs reactivate some sort of demon pact which allows them to defeat him by giving them Chaos damage.
- Also, Illidan devours the Skull of Guldan (another VBA), which the Legion were using earlier to spread corruption in the Night Elves' forests. This turns him into a half-demon, but gives him the powers to defeat Tichondrius.
- Someone told Prince Arthas that to defeat Mal'Ganis (the dreadlord), he must search for the Frostmourne (a legendary sword). Once Arthas gets it, he confronts the dreadlord. Mal'Ganis laughs and states he fell into their trap, and to add to the sucky-ness, the sword consums Arthas's soul. While it's true, Arthas's real master is the Lich King, who seems to serve the Burning Legion and has his personal agendas. Arthas then follows orders to slay Mal'Ganis.
In Neverwinter Nights 2, the only way to injure and even kill the King of Shadows was to find the Silver Sword of Gith. But it was broken into several shards when it was used earlier and your character isn't the only one interested in collecting them.
In the original Doom 3, the player uses the Soul Cube to defeat Cyberdemon. The smaller ones, however, aren't limited to this type of weakness. Also, the Soul Cube powers up by using the souls of fallen enemies. Subverted, in patches.
Legend of Zelda:
- In most of the games, Link needs the Master Sword to defeat Ganon.
- In the Phantom Hourglass, Link needs the Phantom Sword to defeat Bellum.
- In A Link to The Past, the Silver Arrows, which are an upgrade of his regular arrows, only then will Link will be able to use them to defeat Ganon.
In Baldur's Gate 2, you are told that you'll need a special rod of destruction to defeat the Unseeing Eye. Actually, it's easier (though less profitable) to just kill his followers until he appears, since the Unseeing Eye is just a standard mage with more hit points than usual and less dangerous (unlike the beholders that one needs to kill just to get the rod).
In Colobot, the only way to defeat the alien queen is to find and use the Phazer Shooter. You'll need to collect four keys to open the vault in order to acquire it.
The Fire Emblem series uses this trope too many times to count. Even occasionally using the titular Fire Emblems in certain games - which are various objects/artifacts that meet the trope. Here are some examples:
- Fire Emblem Gaiden has its own Falchion, which is the only thing that can kill off Duma once and for all (besides using the Cleric's Nosferatu spell, which may be a bug in the original game).
- Fire Emblem: Genealogy of Holy War has the Falchion substitute in the form of the Naga spell: In-Universe, it's treated as one, but on a gameplay standpoint the Naga spell isn't necessary to defeat the Final Boss. (Good luck beating him without it, though.)
- In Fire Emblem: Mystery of the Emblem, the Falchion, Starlight, and the Binding Shield (aka the Fire Emblem itself), these artifacts together are used to seal away the Earth Dragons when all the Spheres are returned. The Lightsphere is one of these in its own right (a villain beating artifact), since it's the only thing that can nullify the protection of Hardin's Darksphere.
Both invoked and averted at various times in the late, lamented City of Heroes/City of Villains. During some plot arcs you would be given special weapons to use on the enemy at hand -- but you didn't have to use them, and they would remain in your inventory of powers until you used up their charges (which could be months or even years later). A good example would be the anti-ghost weapons you could get in several different arcs, including a villain arc that sent you up against Ghost Widow. In other arcs, though, you would find, receive or craft weapons or items that would immediately vanish with the end of your need for them -- this included things like the grenades from the Lambda trial or the wand which cures the Lost that you would make at the end of the Midnighter Club membership arc.

Western Animation

In Samurai Jack, the titular character's sword is for the most part the only weapon to take out Aku, because it's Made of Good.
In Adventure Time, the gauntlet that Billy owns has the power to seal the Lich. Unfortunately, the bad guy destroyed it before Finn can make good use of it. Finn has to resort to using the Power of Liking Someone A Lot, just to destroy its physical form.
In Scooby Doo Mystery Inc, Scoob and his friends found out that they need to find and use the Heart of the Jaguar to destroy the Crystal Sarcophagus, because Annuaki was sealed in it. During the final episode, the real "Heart of the Jaguar" wasn't the crystal spearhead (which was destroyed earlier), it's The Power of Friendship which binds the gang together. This and teamwork were used and Scooby use the spearshaft to shatter the sarcophagus.