The Oathbreaker

Jaime Lannister: There’s a reason he’s called Kingslayer.

"I'm the bloody Kingslayer, remember? When I say you have honour, that's like a whore vouchsafing your maidenhood."

—Jaime Lannister, A Song of Ice and Fire

In fiction, oaths are powerful things. They bind demons. They bind honour. They create pacts that grant great power. They are things to be respect, feared, and fulfilled.

And then there is The Oathbreaker.

This is the character that has broken a oath of some sort be it magical or mundane and now is forever branded by his misdeed. Sometimes they see this as almost a trophy. Sometimes this is seen as a mark of shame. It almost always brands the character as a pariah until they manage to restore their lost honour.

This is not just a character that has broken a social taboo. They need to have broken something that they have personally sworn. May be the result of a Frequently-Broken Unbreakable Vow.

Often follows from someone saying I Gave My Word, Blood Oath, or Heroic Vow.

Examples of The Oathbreaker include:

Comic Books

In Planet Hulk, Hiroim the Shamed is a member of the Hulk's Warbound, sworn allies for life. Hiroim, however, was ostracized from his people for breaking a previous Warbound pact, and accordingly given the appellation of "the Shamed" to forever mark his treachery.
In Sandman by Neil Gaiman: "As this blood is shed, so spills your blood, Ruthven Sykes, adept of the 33rd, whose secret name is Ararita... Traitor and Oath-Breaker." Cue skull implosion.

Literature

Jaime Lannister from A Song of Ice and Fire; the broken oath and consequences thereof define large chunks of his character, as well as earning him the nickname "The Kingslayer". He killed Aerys II Targaryen after swearing to protect him, and even though Aerys had a nickname of his own ("The Mad King") he is treated like the lowest of the low by most of the nobility, even in a Crapsack World where people like Gregor Clegane, Qyburn, Joffrey Baratheon, and the Bolton family exist.
- Those that leave the Night's Watch are condemned to death for breaking their vows and deserting The Wall. Ned Stark tells his son that there's nothing more dangerous than an oath breaker whose life is now forfeit. They will do anything to survive.
- Lord Frey is generally viewed with suspicion because he is infamous for not picking sides until the odds are heavily stacked, and his late arrival to a battle to defend his liege lord earned him the In-Series Nickname "The Late Lord Frey." However, House Frey as a whole stepped it up several notches when they violated the law of Sacred Hospitality at the Red Wedding.
- There's also Theon Greyjoy, who gets the unflattering nickname of "Theon Turncloak" stapled to him following the events of the second book and is generally treated as the lowest of the low.
- Jorah Mormont is treated as this by Daenyrs once his spying for Lord Varys is revealed. In the fifth book, he attempts to capture and bring her Tyrion to win her favor again.
Marietta Edgecomb from Harry Potter, told the location of their secret hideout to Professor Umbridge after signing a magical contract claiming she wouldn't do that. As a result she had the word "SNEAK" appear on her face made out of pimples and was shunned by her classmates.
- Not to mention Peter Pettigrew.
Merry Gentry's cousin Cel has broken his oath. It's a huge scandal because among the fey this carries a death sentence, but everyone is so afraid of his mother that they don't do anything about it.
- Merry also becomes head of the Wild Hunt for a night in order to punish an oathbreaker.
In typical fashion, the Vorkosigan Saga has an example of this, but it's not that simple. Ekaterin's husband was killed in a accident immediately after she told him she was leaving him. Because she never went through with the divorce her honor remains intact in everyone else's eyes; but she knows she's an oathbreaker, and suffers the shame of it.
- Miles himself qualifies, for the events in the first part of Memory. He also provides a more balanced perspective: sooner or later, "death before dishonor" means everybody is either dead or forsworn.
The Dead from The Lord of the Rings. Isildur cursed them when they swore to help him fight and then refused; three thousand years later, they break the curse by helping Aragorn—the Heir of Isildur—instead.
The novel Oathbreakers, from the Heralds of Valdemar series, is about the heroic duo, Tarma and Kethry, avenging the murder of the leader of their mercenary company at the hands of her brother, the king of Rethwellan. When they find out what he did, they invoke the Oathbreaker's Curse on him and enact some spectacularly karmic revenge.
In The Dresden Files, Harry has a literal Fairy Godmother. But... this is The Dresden Files. It's not what you think. He made a deal with her a long time ago that says that she can now do with him as she pleases - and it turns out that that is to transform him into one of her hunting dogs. He's had to dodge her attempts to collect on his debt. However, it turns out that she really does want to him safe since she made a deal with his mother, and part of her reason for wanting to transform him is to keep him safe at her side.
Nick Seafort from the Seafort Saga broke an oath to save his ship from a Well-Intentioned Extremist. Although other people see nothing wrong in his actions, he considers himself damned to hell for it.
In the Discworld novel Jingo, "71-hour" Ahmed got his nickname from violating Sacred Hospitality and killing his host (Klatchian Sacred Hospitality lasts for three days, or 72 hours). He wears the nickname proudly as a way of inspiring fear and distrust, which is a good thing for him since he's a Cowboy Cop and being feared by criminals is a definitive bonus. As for the man he killed, he was an admitted mass-murderer.
The man who's name is not Jack Bannister apparently got quite rich by being one of these, according to Fisk in the Knight and Rogue Series.
In the Chronicles of Prydain, breaking oaths is one of Arawn's most infamous habits. If this guy makes a deal, he WILL break it. No matter how little it might cost him to keep it. Or how much more dangerous NOT keeping it could be. And SOMEHOW, there are always more idiots willing to make deals with him.

Oral Tradition, Folklore, Myths and Legends

As The Other Wiki says, the most commonly accepted etymology derives the word "warlock" from the Old English waerloga meaning "oathbreaker" (from waer "promise, agreement" and loga "deceiver").
Ironically, Oathbreaker is one of the names of Odin,^{[please verify]} head of the Norse gods. Given that intangible things like vows are supposed to be impossible for gods and the like to break^{[please verify]} (the god wolf Fenrir was bound by a ribbon made from such ingredients as the root of a mountain and the beard of a woman), the fact that Odin can do that is rather frightening, especially when he can extract vows from everything else in the world and expect them to be kept^{[please verify]} (like when he^{[please verify]} made all the things in the world, save one, give an oath to never hurt his son Balder).

Tabletop Games

In Deadlands: Hell on Earth, Oathbreaker is specific disadvantage sykers can take. It means that they have broken 'the Oath of Unity', a promise to never attack a brother syker (generally interpreted as a syker from the same unit or one of its allied units).
Changeling: The Dreaming featured Oaths quite heavily. Characters who made oaths gained sizable bonuses, but those who broke their oaths lost far more. An oathbreaker was also heavily ostracized, and an entire noble house of The Fair Folk were treated as pariahs because they had broken a forgotten oath in ages past.
- Changeling: The Lost also puts heavy weight on oaths, but for different reasons. Breaking an oath earns you a measure of disrespect in changeling society, likely has tertiary consequences if you swore it on something important (e.g., your faith or your fortune), is a sin against Clarity... oh, yeah, and your Keeper is perfectly aware that you did it, and may likely be hobbled by the conditions of the broken pledge.

Video Games

Cassiopeia of League of Legends was once a beautiful human woman who served as a spy for Noxus by seducing foreign diplomats. However, upon breaking an oath of secrecy to a certain Freljord noble, she was cursed and transformed into her current snake-like form.
Ganondorf from The Legend of Zelda: Ocarina of Time is said to have sworn an oath of fealty to the king of Hyrule before the events of the game, but the young Princess Zelda isn't fooled and knows that he plans to betray the kingdom and seize the Triforce for himself. That ends up being exactly what happens, and over the course of seven long years, he transforms the prosperous kingdom into a monster-infested hellhole, and is implied to have murdered the king he pretended to be loyal too.
- His incarnation in Tears of the Kingdom is an oath-breaker as well due to the game's backstory serving as a Whole Plot Reference to Ocarina of Time. While some details differ he murders the queen this time, and is after her sacred stone as opposed to the Triforce the overall story beats and motivations for oath-breaking are basically the same.
Octopath Traveler has a pair of these as major antagonists in two different storylines.
- The overarching villain of Olberic's story in Octopath Traveler is Erhardt, a once-loyal Knight of Hornburg who betrayed and killed his liege lord during battle. While Olberic is deeply hurt to have been betrayed by his sworn brother, he's even angrier that Erhardt broke his oath and murdered the man he swore to protect. Things get complicated, however, when it turns out that Erhardt had sympathetic reasons for betraying the king... namely, that the king allowed his homeland to fall into ruin (or at least, he genuinely thought he did) when invaders set out to attack it.
- Albus (from Primrose's route) is just as vile as his fellow "Crow Men", but instead of being a sex slaver like Rufus or a sociopathic ruiner of lives like Simeon, he broke his oath as a member of Noblecourt's city watch and helped murder his friend Geoffrey Azelhart purely out of greed and misplaced cynicism.

Web Comics

Eugene Greenhilt from The Order of the Stick went through his life leaving everything he ever started only half finished. The Blood Oath of Vengeance he took as an (ex-)apprentice is the first commitment of his to catch up to him. Leaving the oath unfulfilled by the time of his (permanent) death resulted in him being stuck on the wrong side of the heavenly gates in his afterlife. He spends most of his time in the series harassing his eldest child Roy to complete the oath for him by proxy. When Roy dies, he gets let into the Seven Heavens because he actually tried to fulfill that oath... which pisses off Eugene to no end.
In Girl Genius, Captain Vole is the only person who underwent the transformation into a Jagermonster who has broken his oath of Undying Loyalty to the Heterodynes. As a result, he is no longer considered a Jager by the others.
- Nor by himself, as he calls the other Jagers weak and takes pride in no longer being one of them.