Instant Allegiance Artifact

An Instant Allegiance Artifact is, in short, an artifact that will change the alignment of a character. This could be with voluntary use of the artifact or be used by someone else to change alignment.

A variation is that the artifact does nothing at all, but the subject thinks it does, and behaves accordingly.

See also The Dark Side Will Make You Forget and Hypno Trinket. Compare Mirror Morality Machine. For artifacts that instantly ally to their users instead of the other way around, see Loyal Phlebotinum.

Examples of Instant Allegiance Artifact include:

Anime and Manga

Yuuichi Kirisaki's Go-Pan #1 caused an instant alignment change after consumption.
In the first season of Digimon, there were the Black Gears, followed by the Black Rings, Black Spirals, and Dark Spores in Digimon02. The first three turned Digimon evil until they could be removed or destroyed, while the Dark Spores turned humans evil.
In Fairy Tail, the ancient magic Nirvana did this, with a slight twist in that it turned villains good as well as turning good people evil. (as long as their emotions were in flux: basically, a good person would turn bad if they were depressed, angered, or otherwise in a negative state, while a bad person would turn good if they were happy, pleased, or otherwise in a positive state.)

Comic Books

Spider-Man's black symbiote costume typically brings out the darker, selfish desires of whoever wears it.
Many forms of kryptonite have this effect on Superman.
The Thinker helmet in Suicide Squad grants the wearer technopathy and massive intellect boosts - however, the darker facets of their personalities are left to take the helm.

Film

Letting The Golden Child touch you will result in you becoming his protector, even if you were ordered to kill him.

Literature

Possible Ur Example would be the One Ring.
In The Witch's Eye, from the Witch series of books, the titular glass-eye seems to make anyone who carries it slowly turn evil. And, much like The One Ring, it also makes it very difficult for the carrier to throw it away.
In Fred Saberhagen's Book of Swords book series about a set of magic swords forged by the gods, one of the swords - the Mindsword - had this as its ability. Whenever it was drawn, anyone who saw the blade would instantly become a follower of the wielder. Let me repeat that: ANYONE who saw the blade, even the gods themselves, would instantly become a follower of the blade's wielder, regardless of who the wielder was, or what the personal beliefs of the victim were. And this could be done just as effectively to entire armies as it was to small groups.
In the Sword of Truth series, the Confessors were basically walking, talking versions of this. They have the power, through touch, to instantly turn anyone into a servant of the confessor. However, there's a restraint to this power: it has to recharge, and it only can do this to whoever the confessor is touching, so when the Big Bad starts hunting confessors, he sends out Quads, groups of four well trained killers, so that the confessor can change one of them, the other three can kill the changed one, then the survivors kill the confessor afterward. It's a highly effective method, as long as they can catch the confessor alone.
- Before the Big Bad made the scene, confessors were used to obtain confessions from suspected criminals, since after a single touch, the victim would tell the truth, the whole truth, and nothing but the truth on command. However, since a confessor's touch is essentially permanent, they would only use it on people who's guilt was almost completely certain, in order to confirm their guilt before execution. Unfortunately, the person who SEEMS to be the guilty party isn't always so, so not every person whom the confessors touched was guilty.
A rather odd example is the Horn of Valere, which summons the spirits of heroes bound to the Wheel of Time—who will then fight for whoever blew the Horn, regardless of their allegiance and moral alignment.
The Caps in The Tripods.
In Helm, this is one of the uses of the imprinters.

Live-Action TV

From the Stargate SG-1 movie Ark of Truth the aforementioned ark is speculated as being one of these, with the ability to instantly change someone to the side of truth. Turns out it's basically programmed to show followers of the Ori that they're wrong, but it's speculated that it could be reprogrammed to make someone believe anything.
Legend of the Seeker has the Boxes of Orden, which, when put together, give the wielded the power to instantly gain the allegiance of anyone in line-of-sight. In order to counteract the seductive effect of Orden on the wielder, the activation must be coupled with Confession. Breaking up the boxes instantly returns all who were under their power to their original allegiance.
- An attempt by Zedd to alter Cara's past results in a radical change to the timeline with Richard succeeding in using the Boxes of Orden to turn Rahl into a loyal ally and become the ruler of D'Hara.

Tabletop Games

In Dungeons & Dragons some artifacts have curses that gradually change Character Alignment.
- Almost this, the Helm of Opposite Alignment from will flip the Character Alignment of any who wear it to its diametric opposite. It doesn't switch allegiances immediately, but it can potentially unmake any paladin who wears one or tear an adventuring group apart... though that might be debatable, considering the ethical standards of some D&D players.
  - In earlier editions the change took place immediately.
  - In Dark Sun this little trinket led to the founding of the Veiled Alliance—a high-ranked templar got subverted and used his penchant of weaving conspiracies for a good cause (Water and ashes short story by Allen Varney).
- Forgotten Realms has the Crown of Horns, enchanted by Myrkul, former god of death, to turn the wearer's alignment to Neutral Evil. The first Crown came to control a high-ranked divine minion and was promptly crushed by another servant of the goddess it crossed. If it wasn't bad enough, the Crown v.2 contains what's left of Myrkul, and the wearer is stuck with it not until some priest manages to get close enough to cast "remove curse", but until the dead god chooses another puppet. While gradually transforming into an undead. That will turn to dust once Myrkul decides to leave.
- The god Cyric created a magical book that would convince everyone of his omnipotence and perfection. When he read the book himself, he became completely delusional.
- When the Eye of Vecna is placed in a living creature's empty eye socket, the creature will immediately change alignment to Neutral Evil. If the Hand of Vecna is grafted onto a creature's arm stump and one of the Hand's primary powers is used, the creature will become Neutral Evil at once. (In earlier versions, this has the added bonus of making the Eye and Hand's wielder think they are actually Vecna.)
- Those unfortunate enough to look into the Mirror of Simple Order (from Tome of Magic) long enough are polymorphed into their reflection, which is grayed out and stripped of ornaments. For really unfortunate ones this also includes turning Lawful Neutral with very boring personality, average stats, and out to set order to the world.

Video Games

The Warcraft series contains the artifact Frostmourne, which sucks out the soul of anyone who uses it and lets them hear the voice of the Lich King, thus equipping them with a lack of conscience and a new master in one stroke.
Nethack got a helmet that temporarily causes your alignment to flip.
- Bizarrely, it doesn't make you do anything per se. It just causes opposite damage. So with some exceptions, you can put the helmet on, do some stuff that normally causes Karma damage, uncurse it off, and it translates as if you've been doing the proper actions.
  - Be careful about sacrifices, though - do the wrong ones, cement your alignment, and you've triggered the one way to make Nethack Unwinnable.
Arcanum: Of Steamworks and Magick Obscura has a dark helm which gives the wearer a boost to their magical aptitude in exchange for penalizing (or rewarding, depending on your character) their alignment with 20 evil points. In an unpatched version of the game, both changes are permanent, and a new bonus is added each time the helm is taken off and re-equipped.
Baldur's Gate: Tales of the Sword Coast. Helm of Opposite Alignment, complete with a stick-to-your-head curse!
In Fable clothes or armor have alignment change stat. A full suit accounts for about 1/4 distance between pure good and pure evil. If you feel you're too nice, just wear evil clothes, and you're a 50% less good anti-hero.
The Psychosis Gun in Perfect Dark makes the target think friends are enemies and vice versa. It figures most prominently in one of the bonus missions that Elvis undertakes.

Web Comics

In Erfworld, the Arkenpliers, as wielded by their attuned master, are capable of Decrypting units (which works like Uncroaking/Animate Dead, except they resurrect as sentient, non-decaying units instead of zombies). The Decrypted are loyal to the Arkenpliers' user regardless of past loyalties; even the Decrypted Prince Ansom experienced a complete alignment reversal. It was initially unclear whether that was because of the Arkentool or just a matter of him going nutty over being beaten by Parson, but it has since been revealed that the Arkenpliers have that effect on anyone.
- Not so much an alignment reversal (he retained good intention) as a ideology switch (he came to see royalty as wrong and became a toolist).
In Eerie Cuties, Chloe's mom has the "Doom Panties" locked away. These enhance a succubus' power as well as their Horny Devil instincts. When Chloe puts them on at too young of an age, she instantly transitions to a full-blown succubus and goes from a nice, nerdy, shy girl to contender for Alpha Bitch.

Western Animation

In Kim Possible, there was a helmet that flipped people between being a superhero and supervillain.
- Later turned into a laser weapon, causing multiple midfight switches. And of course, it hits Ron.
The Ying Yo-Yo and Yang Yo-Yo in Xiaolin Showdown. If you use one to go into the Ying-Yang world, you come out evil. (A later episode says it just reverses alignment.) Using both nullifies this drawback.
Megatron infamously reprogrammed Rhinox in the Beast Wars episode Dark Designs. The latter's strength and intelligence, coupled with Predacon ambition and lack of scruples, allowed him to singlehandedly trash half the base and take over the rest before he picked up the Villain Ball and Megatron changed him back.
- Also done to Ravage via a recording of the G1 Megatron
- And this is what G1 Autobots did to Devastator.
  - Which is only fair as a later episode revealed Megatron made the Constructicons evil.
The Go Bots had a Good/Evil switch of sorts. Why do they even have that button?