In many stories with fantastical or sci-fi elements, there's one particular object or weapon that the heroes rely on — perhaps a Cool Sword or Orphan's Plot Trinket that just so happens to be the source of their powers. It will always be stated as innocent or innocuous at the least, and outright holy at the most extreme. In both cases, this is the hero's trademark weapon and the crutch by which they battle and carry out their adventures; if it's of the sacred variety, the heroes may use it as a crutch for the morality of their actions, too: Sure, it might leave a bad taste in their mouths to have killed the sympathetic Anti-Villain or the Tragic Monster, but they have their holy sword! They must have the forces of Good behind them, so it's all right!
In a case like this it's not that pretty—this weapon or object works through evil means. Perhaps it's Powered by a Forsaken Child, or perhaps it's a keystone in the villain's Evil Plan, and he's manipulating the heroes into handing it over to him or powering it up for him.
The Reveal of this information is expected to be proportionately dramatic to the item in question's plot importance. Depending on what point in the plot the truth comes out, a variety of outcomes can occur:
If the truth comes out early in the plot, viewers can expect a brief Heroic BSOD or 10-Minute Retirement in which the heroes attempt to cast aside the item in disgust. Sometimes they can find another power source, although usually they are persuaded to keep using it until the world is saved, when they can stop using it safely.
If the Unholy Holy Sword is plot-important or The Reveal occurs during the climax or penultimate stage of the story, the plot may very well shift in the direction of sealing or destroying it so that no one can use it again, if not a full-on dramatic confrontation with the villain over its true purpose.
The 13 Months from Tower of God are a set of powerful empathic weapons given to the Princesses of Zahard. Given what we've seen from Black March, they seem rather benign and weapons of good, but slowly information is trickling through to indicate otherwise; for instance, how the Eurasia family refused to be part of the Princess system any longer after their most powerful Princess, Eurasian Enne Zahard, went berserk after she received her sword.
The Sword of Light in Slayers was originally thought to be a weapon of good, passed down in Gourry Gabriev's family. It was originally used to defeat the Demon Beast Zanaffar, and was seen as an object of great good for a long time by the entire world. Then, in Slayers TRY, it was revealed to be part of a set of five similar weapons from another dimension, which are that reality's version of the Five Dark Lords.
In Night Wizard, the seven Jewels of Virtue the heroes are collecting turn out to be fragments of (God of Destruction) Shaimal's power. Once the Jewels are brought together, Shaimal begins to awaken within Eris, who's horrified upon learning that she's just a vessel for Shaimal's reincarnation.
The Evangelions from Neon Genesis Evangelion are touted as the only weapon capable of protecting the world from the Angels and The Third Impact. Turns out that A) They're cloned from angels themselves and powered by human souls, and B) They were created to initiate Third Impact... on SEELE's terms.
The Red and Orange Power Rings in DC Comics (specifically the Green Lantern books). Not only are they powered by rage and greed, respectively, but their wearers become consumed with those emotions as well. Yellow Power Rings are fear-based, but their wearers generally retain their personalities.
The Violet Power Rings are powered by love and cause their wielders to become obsessed with it, though not to the extremes of the original Star Sapphires. This is partially due to the brainwashing most recruits go through before they are taken into the Corps.
In Buffy the Vampire Slayer, it turns out the power of the Slayer line began when the First Slayer was imbued with the power of a demon.
In the movie Feng Shui, the lead character receives a "lucky" ba gua mirror. It is in fact lucky, in the superficial sense, but it also turns out everyone who looks in the mirror dies according to their Chinese zodiac sign.
The Disney/Pixar movie Monsters, Inc. uses the screams of small children to power... well, everything. Nobody thinks this is bad, as they're convinced children are toxic. After inadvertently scaring Boo, Sully finds a new source of energy: children's laughter, which is 10 times more powerful. Also, children aren't really toxic, just very energetic.
A more symbolic version in Star Wars. Luke receives his father's lightsaber, which helps inspire him to become a Jedi. We later find out that the same weapon was used to slaughter children.
In the Inheritance Cycle, the sword Eragon gets from Brom, Zar'roc, is later found out to have belonged to the last of the Forsworn, Morzan, who was killed by Brom.
Over the course of The Hobbit and The Lord of the Rings, the One Ring is treated something like this — it's Bilbo's most prized artifact and gets him out of a lot of trouble, but once its true nature is exposed, it takes a narrative six times longer than the first book to destroy it safely.
Gurthang, the sword of Túrin Turambar in J.R.R.Tolkien's The Silmarillion. It enables him to slay the dragon Glaurung, but it also claims the lives of many of his friends and loved ones, and in the end his own. (It's a very hungry sword and it will drink the blood of anyone, including its wielder.)
In Memory, Sorrow, and Thorn, the eponymous swords the protagonists believe they need to gather to stop the Storm King are actually the key to his return. This becomes the third version, as, after the reveal, the focus of the story abruptly shifts to how to stop the bad guys from using them. Of course, they are too late, forcing the heroes to Take a Third Option.
In The Night Angel Trilogy, the ka'kari, particulaly the black, act as this. While not evil, the ka'kari is central to most of the villains' plans, as it grants immortality via stealing the life energy of the bonded person's loved ones everyt ime the user dies. Needless to say, when Kylar learned of it, it was already bonded with him and he had died 5 times already.
Mostly only the black. The other ka'kari are just insanely powerful and are never presented as benevolent. And even the black is never truly evil, and is just the cost of immortality. Unfortunately, the letter Durzo left for Kylar was damaged and Kylar did not know of the cost until it was too late. In fact, the Black Ka'kari itself repremands Kylar. And the most Ka'kari goes out of its way to delay the final sacrifice of the book so as to give Kylar a chance at a brief moment of happiness. And it resurrects Durzo as a freebie so that he too can live out the rest of his years in peace and happiness.
In The Secret Histories novel The Man with the Golden Torc by Simon R. Green, it's revealed that the golden armor that the Drood family wears, that makes them super powered and indestructable, are powered by the souls of their dead twins which are consumed by The Heart (a giant, living crystal with godlike powers) shortly after birth.
In The Secrets of the Immortal Nicholas Flamel, the elemental swords are well known — one is Excalibur. They also all actively consume the memories of those wounded by them, and eventually the minds of an unprotected wielder, and can potentially destroy the world if united. The heroes use one. Oops.
The Subtle Knife from Phillip Pullman's trilogy, His Dark Materials. The knife can cut through dimensional barriers and enables the majority of the plot to happen. However, not only are the holes it creates drawing out the Dust that powers all sentient life in the universe, but using it creates horrific monsters that eat the soul of any adults (ie, Dust magnets) they find.
The Elric Saga: Stormbringer is a decided aversion, though it probably needs to be mentioned because it keeps popping up on this page anyway. While the sword does provide Elric with much of his strength and proves as effective when used against the forces of Chaos as against most other things, nobody even passingly familiar with the weapon is fooled in the slightest into thinking that it is not evil.
Retconned a bit as Moorcock's multiverse develops further. The Black Sword(s) (of which Stormbringer and its twin Mournblade are incarnations of) were actually made to fight against Chaos and any other threat to the multiverse. It's just that the Melnibonean Emperors used them to fight anything else as well. Stormbringer isn't that much of an aversion to this trope, it did have a revelation. The big revelations were in finding out how important it is to the cosmos (instead of being just a really hard-hitting magic sword) and just how evil it truly is (Elric underestimates this to great cost).
Disturbingly implied in The Way Of Kings. Shardblades are the only weapons humanity possesses that can harm the demonic Voidbringers, but when you discover that Sylphrena, a being literally Made of Good, gets antsy around them and thinks Dalinar is a better man for getting rid of his... The truth is revealed in Words Of Radiance. As it turns out, Shardblades are spren like Syl, who learned how to take weapon form through their bonds to the Radiants. When the Radiants broke their oaths, the bonds also broke, killing (in a manner of speaking) the spren they'd bonded to. Syl's revulsion to the shardblades is because they are the bodies of dead spren.
Yggdra Union has the Gran Centurio, which just so happens to be the Trope Namer. For centuries, it was believed to be a symbol of justice incarnate. However, it was eventually revealed to be a power generator that a manipulative fallen angel planned to use as the instrument of his revenge against the gods. It so happened to run on people's hatred and pain.
Exspheres in Tales of Symphonia. Although The Reveal happens early, Exspheres and the ramifications of using them continue to feature in the plot even after the heroes are persuaded to keep using them for now.
The backstory of several Korean characters involves their trying to find the "Sword of Salvation" in order to drive out the Japanese. At first they thought it might be Soul Edge, the MacGuffin pretty much everyone else is after. After seeing its evil true nature in Soul Calibur II, they spend the rest of the series warning people of its manipulative nature, and trying to destroy it with Soul Calibur. Then Talim makes them realize Soul Calibur is just as bad.
Sudeki; the first half of the plot revolves around collecting crystals to power a "Peace Shield" around the kingdom of Haskillia, protecting it for all time. As it turns out, the crystals actually gain their power by sucking the "light" (akin to Life Force) from the mirror dimension of Akloria, leaving it to essentially rot as Haskillia blooms. To say nothing of the machine's real purpose, summoning Heigou...
The Seru in Legend of Legaia might be borderline in the backstory, seeing as they're living armor and weapons that symbiotically bond to their human wielders... and have an unfortunate propensity for getting Brainwashed and Crazy and taking those wielders with them. The entire plot of the first game involves the heroes trying to fix a world terrorized by insane Seru.
Frostmourne in Warcraft III. It was originally the sword of Ner'zhul(the Lich King), which he tricked Prince Arthas into using (which he did, believing it was the only way he could defeat the plague of the undead), while its true purpose was to corrupt Arthas into joining him.
The Ashbringer zig-zags around this trope. The crystal that it was made of started as evil, but was purified. The sword itself was created as good, later corrupted, and then purified again. And then there are theories that the crystal is somehow related to Naaru, or is one, going through their dark/light reincarnations.
Quel'Delar, the Infinity–1 Sword of World of Warcraft: Wrath of the Lich King, goes through this stage briefly during the quest to reforge it. When it's been reforged with saronite, an ore that is actually the blood of a major Eldritch Abomination (Made of Evil, of course), it draws its owner to the vicinity of Frostmourne and attacks on its own accord. After that, it too needs to be purified.
The Grandlion/Masamune in Chrono Trigger is a complicated example. It's first mentioned as a heroic knight's blade, but turns out to have been filled with the power of Lavos, but is also powered by a pair of playful spirits that react to people's dreams. So it goes from good, to bad, to neutral, to good again. By Chrono Cross, it's been tainted again (by a powerful person wielding it for evil), but you can fix it.
Chrono Cross also has the Records of Fate, which are revered in Arni as if they speak some divine and benevolent will, but it turns out they're used to control the islanders by the aptly-named supercomputer FATE.
The Masemune sword from the original Chrono Trigger was born of the Big Bad's energy, but is heralded throughout history as the sword of heroes. After being abused for evil purposes, it reverts back to its original red hue and becomes malevolent.
Brave Fencer Musashi features Lumina, the sword of luminescence, which was used to defeat the Wizard of Darkness in the distant past. Musashi claims it early in the game, and spends most of the rest of it finding and claiming the Five Scrolls that increase its power. Naturally, upon acquiring the final one, it turns out that Lumina wasn't just used to defeat the Wizard of Darkness, the Wizard of Darkness was sealed inside of it and the scrolls have been breaking the seal.
Arcadion the Shade Sword, or The Black Sword, from Ultima VII: The Black Gate (the Forge of Virtue expansion). Forged from a supernatural metal and empowered by its captive demon, it is a superb weapon that can kill anything that is capable of dying, possesses many other potent powers, and the demon can offer plenty of advice on matters. On the other hand, it is a demon-powered sword and you're supposed to be the Avatar, a champion of Virtue. It's fairly far-fetched for you to want to wield the thing, or forge the cursed thing in the first place. You can avoid it by not completing the expansion, but since you start with it in Serpent Isle, Arcadion is canon.
Heroes of Might and Magic 4 has the Angel's Blade. It's not necessarily evil, and its primary function is killing undead and demons, but a quest during the campaign reveals it was created by a demon. It doesn't really matter since 1) the main character is a necromancer fighting other demons, and 2) he only needs the sword because the energy released when destroying it will open a portal to where he needs to go.
The last campaign of Heroes Chronicles revolves around the Sword of Frost, an extremely powerful sword which is being sought by the Barbarian King Kilgor as a counter to Gelu's Armageddon's Blade, by Gelu to destroy Armageddon's Blade and keep the Sword of Frost out of Kilgor's hands, and by Tarnum to both keep it out of Kilgor's hands and keep Gelu from destroying Armageddon's Blade in the wrong way. Sadly, the wrong way that the Sword of Frost ends the terror of Armageddon's Blade is what happens... and it is by blowing up the world, leading to the events of HOMM 4.
The Elder Scrolls IV: Oblivion has a variation, where there is a sword that is holy (wielded by a powerful time traveling berserker-saint who was a robotic xenophobic personification of a missing god, no less) that was cursed afterwards by someone who used it in a war for material conquest.
The Neverland games by Idea Factory invert this repeatedly with a whole slew of weapons with more or less ominous names that turn out to be potent forces of good. Among others, Demon Princess Hiro's Gate of Hell scythe can sunder someone's soul from their body and send it straight to Hell, but only works on those with an extremely evil spirit, and the Evil Spear Aleph-Beth was the weapon of choice of Arth, a holy angel who ascended to godhood and, in spite of his sudden fit of rage against humans at one point in history, was actually one of the few ascended gods who WEREN'T homicidal maniacs.
Kite's Bracelet from the .hack R1 games was the heart of Cubia, the most dangerous enemy the party had ever encountered, and needed to be destroyed before said enemy could die.
The Monado in Xenoblade. It's valued by the Homs for being one of the few things able to harm the dreaded Mechon, and is used by Shulk to save the lives of his friends numerous times, thanks to its abilities to let him see the future and effectively warp reality to some degree. Unfortunately, it turns out to be containing the soul of an egotistic god who intends to eradicate all life and recreate the world, and the only reason said god hadn't taken control of Shulk was because he was unknowingly following his plans for almost the entire story.
Dark Souls features Artorias the Abysswalker, who turned his signature greatsword, which had divine properties, into a cursed blade capable of harming the ghosts residing in New Londo. His sword is tainted again in the Downloadable Content, this time by Manus, turning it into the Abyss Greatsword.
The aptly-named Sword from the yuriVNAoi Shiro. It was used to summon the divine storm that sunk the invading Mongol fleet in the past. Partially subverted, as those with qualifications to use it know that it's holy in the sense that "this item with terrible power is a property of the gods, do not touch," not "this is a weapon to battle evil." Also, it's not mentioned as such during the game itself, but it's theKusanagi. It was originated from the Chaos that people in the past called Orochi, and will definitely, without exception, corrupt its user.
In the third route of the Visual Novel, there's also Saber's Excalibur. For the first two routes, it is a holy "Sword of Promised Victory" that fires off a beam of bright light. Halfway through the third route, she is corrupted by the evil in the Grail and becomes the newly villainous Sakura's Servant. The switch from Lawful Good to Lawful Evil is accompanied by her sword turning into the pitch-black "Excalibur Morgan" and firing off red/black beams. This is explained as being due to Morgan Le Fay's hand in its creation.
The prequel light novel and anime Fate/Zero has Arondight, Berserker's actual weapon. It used to be a holy sword that would never be damaged, made to be wielded only by a perfect knight, but when her wielder Lancelot went mad and used it to slay other Knights of the Round, it became a demonic sword filled with the madness and resentment of its master.
The Alternate Universe novel Fate/Apocrypha has Clarent. Originally it was a sword of peace, made not for battle but for knighting and ceremonies, but was transformed into a demonic sword by Mordred's wanton hatred and used in rebellion against the lawful king, practically becoming a blood-based Excalibur.
Inverted in Goblins by the Axe of Prissan. It was designed to generate an aura of evilness, so that any paladins who are aware of it would seek it out in order to destroy it. Once they touch the axe, it telepathically communicates to them that it's actually being used to imprison a powerful demon who once tried to invade the Prime Material Plane, and that a paladin must constantly use it to do good deeds, which will strengthen the wards of the demon's prison and prevent it from escaping.
Furthermore, it is not only incapable of harming a paladin — passing through them without leaving a wound — but it also provides its wielder with a suit of magical plate mail, to better aid the above-mentioned good deeds.