Supernatural powers can come from a lot of sources, and often come at a price. Given that the receivers of these powers have great ability, and great temptation, to misuse them, it makes sense to put certain behavioural conditions on them.
An Oathbound Power is one which depends on the adherence of its wielder to a particular creed, ethos, set of laws, virtue, trait, etc. Typically this means a Heroic Vow, Badass Creed or similar, though it can also be one of evil or cowardice. Fundamentally, this ensures that anyone receiving the power is worthy of it, and will remain so. Such a person is necessarily The Fettered, and though they may view their oath as a Restraining Bolt, they are technically free to forsake it. However, and crucially, doing so means losing the powers associated with the oath. A common use of this trope in a plot is to have someone break the oath and lose the powers, in order to learn a lesson, reclaim their virtue, and regain the powers again.
Powers of this type are typically given (or, possibly, lent) by a powerful supernatural being, via a geas or Magically Binding Contract. Alternately, it may be that an Empathic Weapon can sense the temperament of its user and decide whether to cooperate. Another alternative is for the limitations to be self-imposed by the user for one reason or the other (e.g. fear of getting Drunk On Power).
From a writing point of view, this restriction can serve to quickly demonstrate the value of a hero to the audience: we know this character is good because they can do that special thing. Likewise, any morally ambiguous deeds they perform must have been justified because their powers aren't diminished, or at least so the author asserts. Similarly, anyone holding to an evil or distasteful oath can be assumed to be up to no good.
Frequently, the powers are activated by reciting the oath. This is By the Power of Grayskull! See also Situational Sword, Conditional Powers, Geas and Phlebotinum-Handling Requirements. Often overlaps with Only the Pure of Heart.
- In The Twelve Kingdoms, rulers are chosen according to the ineffable will of the gods, giving them immortality and authority over the kingdom, including command of a cadre of supernatural creatures who can give them other enhanced abilities such as uncanny martial prowess. Yet if they rule poorly and the country suffers, their royal beast the Kirin comes down with a divine illness and both die. Once this begins, there seems to be no way to stop it save through abdication.
- Hunter × Hunter combines this with Conditional Powers. There is a mechanic in Nen that allows a Nen-user to greatly strengthen their abilities by adding "vows" and "restrictions". These work by giving a specific power or powerset a limitation (such as, it will only work against a specific person, at a specific time) and vowing that they will honor that restriction at risk of some immediate and devastating consequence (such as death). The greater the restriction, and the greater the consequence for breaking it, the stronger the ability will be.
- Played for laughs in Scott Pilgrim. One of the evil exes gets his telekinesis and other powers by being vegan, so when he's tricked into drinking real milk, he's stripped of his powers (on a third strike, no less.)
- In The Black Ring, Lex Luthor does a Fusion Dance with a being called the Zone Child, gaining tremendous power. However, the Zone Child ensures that he only keeps that power if he doesn't do anything negative with it, such as, say, destroying Superman. Three guesses if Luthor actually kept the power.
- The eponymous Golden Child has powerful supernatural protection as long as he remains pure, in accordance with Buddhist principles. The Big Bad tries to break this after kidnapping him by tricking the child into eating food that contains blood, to no avail.
- In the film Excalibur, as in many versions of Arthurian legend, Arthur's possession of the titular sword grants him the right of kingship. At one point in the film, he uses the sword for a selfish purpose- to defeat Lancelot- and it breaks. When he realises his error and repents, it is returned to him whole.
- In the Name of the King: According to Merick, a magus must always serve a king in order to retain his power. Gallian serves no one yet is extremely powerful. When asked, Gallian reveals that he had the Krug accept him as their king and now serves himself.
- The Dresden Files
- The Faerie courts of Summer and Winter can bestow the mantle of Knight onto a singular mortal who gains special powers, but who is also bound by the Fey inability to lie or break one's word. At one point after taking the position, Harry tries to go against his word and finds that not only does the power vanish, he also regains the Game-Breaking Injury that he originally accepted the job in order to cure.
- Then there are the Swords of the Cross, which represent the virtues of faith, hope and love. They are normally indestructible and can be used to great effect by one who exemplifies those virtues, but are just low-quality swords to anyone else. Additionally, if an appointed user puts the swords to any unworthy use, they lose their power until this is amended, and can be broken- with no possibility of being reforged.
- In A Song of Ice and Fire only those who have sworn the vows of the Night's Watch can open the enchanted gate in Nightfort. The gate is one of the very few ways of crossing the enormous Wall separating the Seven Kingdoms from the Far North; when Bran Stark and his company enter Nightfort, they cannot get to the other side until a Watchman named Sam Tarly comes to meet them.
- The Knights Radiant in The Stormlight Archive are made up of ten orders who each exemplify a specific virtue. They take Elemental Embodiments called spren as Bond Creatures and swear to Five Ideals, the first of which is the same for all Knights Radiant and the rest specific to each order. Each Ideal they speak gives them additional powers from the spren bond, and forsaking those ideals causes the spren to "die" and the powers to vanish. In the Back Story, the Radiants did this en masse, and subsequently disbanded, in an event called the Recreance.
- Samson of The Bible was bound by the Nazirite vow from birth, in return for which he had stupendous power, with which he slaughtered hordes of Philistines. The vow stipulated that he forgo practices such as drinking wine or cutting his hair. When the deceptive Delilah found this out, she had his hair cut and he was captured. Eventually the Philistines set out to sacrifice him to their god, but by then his hair had grown back, and he destroyed their temple in a classic example of Taking You with Me.
- In the Mahabharata, Bhishma earned the boon of choosing the time of his own death by taking a celibacy vow, so his father, a king, could marry his Second Love, who wanted one of her own children to inherit the throne.
- In Fantasy Hero, priestly magic normally has the limitation "Only When Serving The God's Purposes".
- Dungeons & Dragons:
- Paladins acquire special powers, mostly of the "smiting evil" type, by following the Good Alignment in accordance with their particular god. If they stray from their tenets, they can lose some or all of their powers.
- Clerics are bound in a similar way to paladins in that they are required to follow the precepts of their chosen cause (most commonly church dogma of a deity) in order to retain their powers.
- The Book of Exalted Deeds and Book of Vile Darkness supplements for 3rd Edition feature a number of feats titled "Vow of X" that grant the character various powers in exchange for taking, or not taking, a given action. For example, taking "Vow of Celibacy" grants resistance to charm and phantasm effects but requires the user to abstain from both marriage and sex.
- 1st Edition characters with the monk Character Class have physical and magical abilities and must be one of the Lawful Character Alignments (Lawful Evil, Lawful Neutral or Lawful Good). If their alignment changes for any reason they lose all of their powers and must start over at 1st level.
- 1st Edition Deities & Demigods Cyclopedia. Members of the cleric/priest Character Class gain spellcasting ability through their worship of and dedication to a deity. They must be the same Character Alignment as their deity and follow all of the deity's rules. If they don't they will receive increasingly severe punishments, including loss of their powers.
- In GURPS, any advantage or power can be modified with the Pact limitation, which turns it into this.
- Magic: The Gathering has a cycle of "pacts", one for each color. Pacts are spells that can be cast for free. However, the caster must pay a cost at the beginning of their next turn—or else they immediately lose the game.
- Princess: The Hopeful:
- While Princesses can for the most part use their magic with no particular limitation regarding their attitude, they also have a set of abilities known as Invocations, which allows them to boost most of their Charms and is required to activate part of them. Each Invocation is connected to a particular Queen, and as such requires following their philosophies to properly work; an Invocation can be used for free if the action a Princess intends to do matches the Queen's philosophy, while acting against said philosophy will cause the Princess to lose the Invocation for a specific amount of time.
- Sworn powers work like this; they draw power directly from their Queen, so their magic grow stronger when they act in accordance with said Queen's philosophy and weaker when they act against it. To represent this, they have access to Invocations as described above, and their Wisp pool is equal to their Integrity.