Fairy

Fairies are the living inhabitants of Annwyn. Fairies come in all shapes and sizes, and in various levels of intelligence. Fairies of bestial level intelligence are known as Shee. Those of humanlike intelligence are known as the Tuatha De Danaan, more often just called Tuatha. Tuatha that have not only humanlike intelligence but also mostly humanoid form are known as Elves. But fairies don't have to take the form of animals--they can also appear as plants or as seemingly-inanimate objects.

All creatures of Annwyn are Fairies, and all magical creatures come from Annwyn. This includes creatures that would not normally be considered fairies in folklore or other worlds, such as dragons.

Common Properties
While Fairies can take many different forms and have wildly differing psychologies, there are a few features which are common to all Fairies.

Contracts
Fairy morality is based on Contracts. Although "morality" might be the wrong word. A Full-blooded Fairy is physically incapable of intentionally disobeying a contract, and even unintentionally doing so can create powerful physical responses to prevent it. Those who manage to unintentionally break a contract will suffer immense shame, sometimes to the point of physical illness or pain.

Fairy magic can never violate a contract that the fairy has entered into, intentionally or unintentionally. This also applies to humans using fairy magic through an Artefact, Sorcery or a Wild Talent, even if they suffer no physical stress from breaking the contract personally.

However, fairies do not believe in the "spirit" of contracts. The letter of the law is the only thing that matters. Fairies will never feel bad about violating the "spirit" of a contract even of a human they respect. But likewise, fairies are never offended by a human outmaneuvering them through technicalities in the contract even if it goes against what they desired from the contract in the first place--to the fairy mindset, entering into a Contract that the other person can wiggle out of through clever wordplay was their own mistake, and they deserve to lose out on the deal. In fact this will often lead to them respecting the person who wiggled out of it more than they would if they had abided by the contract--though earning a fairy's respect is not always a positive thing. Even "good" Fairies thrive on mischief, and there is no one a fairy delights more in challenging with their mischief than someone who they respect, if only to see what their limits are.

Morality
Fairies are inherently selfish creatures, and in fact many full-blooded fairies come close to full-blown sociopathy. They are not necessarily prone to malice, but their ability to empathize with humans is practically nil.

Full-blooded Fairies don't really understand human morality without a great deal of time and effort spent studying it. Many humans identify fairies they see as "good" fairies (sometimes called Seelie) and fairies they see as "bad" fairies (Unseelie), but this is often a mischaracterization. A fairy's mindset is alien. As mentioned above, contracts are the primary form that their morality takes. Mostly, "good" fairies are simply fairies who have entered into contracts that make them abide by rules humans would recognize as being close to human morality. There are a few fairies who see personal benefit to playing by human morality without a contract, but these fairies are still incredibly dangerous--they will only continue to abide by human moral rules until they no longer see a benefit, or until the benefit of violating human morality outweighs the benefit of sticking to it. Without a contract to protect them, a human running afoul of such a creature will often be lured into a false sense of security by the fairy's otherwise kind, humane appearance.

Fairies with human ancestors, however, are much more likely to have human moral codes. This does not prevent them from being evil, but it means that the ones who are evil are evil for the same reasons humans are evil, and with the same justifications and psychoses backing them up.

Weaknesses
Individual fairies will often have their own weaknesses, often in the form of things that are anathema to their core concepts--a salamander having weakness to water, for example. However, some weaknesses are common to all fairies. Here are the most common ones:

Cold Iron
All Fairies suffer from an extreme allergy to Cold Iron. This is true not only of their physical bodies but of the magic they use and the magic that gives them life. Cold Iron can break down magical constructs and disenchant even very powerful enchantments, barring clever methods of preventing disenchantment--especially clever Elves and Sorcerers have taken to using powerful electromagnets to prevent Cold Iron from getting close to enchanted objects, for example. But this is difficult to maintain.

Cold Iron is not the same as regular iron. Cold Iron must come from meteors--iron mined from the earth is not Cold Iron. It also can't be part of an alloy--Steel can never be Cold Iron, even Meteoric Steel. It also can't be in part of a chemical solution, so for example the hemoglobin in human blood does not hurt fairies, even if that human somehow got the iron from meteors. Though some people do believe that it's the iron in human blood that ensures that Elves with human ancestry can never be as powerful magically as full blooded fairies, there's no way to prove that. If it is true, it seems to be more an effect of prolonged exposure than anything that could be used intentionally against someone.

Running Water
Running Water also confounds fairy magic, though not all fairies are physically impeded by it. Merrows, Undines, and Nixies for example thrive in running water. There are some fairies that cannot cross running water though, no matter how shallow, and have to find a bridge to ford streams. Fairies that are capable of entering running water are capable of using magic from within a stream or river or the like, but that magic cannot exit the water, and magic cast outside of the stream cannot cross it. It acts as a natural magical barrier. This does not mean that a practitioner cannot affect someone on the other side of a river however--a clever practitioner can find ways around the issue. They just can't make the magic itself directly cross the barrier.

True Names
Knowing a fairy's True Name gives a human (or another fairy, for that matter) power over that fairy. A fairy can be compelled to come when called by their True Name, and it is sometimes possible to force them into Contracts when using their True Name as well. Because of this, Fairies almost never reveal their True Names to anyone unless they have a debt that cannot be repaid with anything short of their True Name, or some other dire need has arisen. The exception seems to be fairies of human descent--among these fairies, it has become a sign of True Love, or of True Friendship, to share True Names with one another. Full blooded Fairies are generally thought to be incapable of either of these emotions, so this is not a trait they share with their descendants.

Even Shee have True Names, though as they are incapable of speech, it is very difficult to learn a Shee's true name. Much like with Tuatha, however, a Shee can be summoned or forced into a contract so long as their True Name is known.

Human Faith, Human Love, and Human Friendship
For some reason, human faith, when wielded as a weapon or a shield against fairies, is powerful against them. This has to be true faith in the hands of a true believer, but someone who has a genuine belief in a higher power--regardless of which higher power-- can turn back fairies and protect themselves from fairy magic. This does not even have to be a god--a human with a devotion to a concept or ideology can utilize the same power, even for something as petty as devotion to money or slavish support of a sports team. Holy Ground and Holy Symbols can work this way as well, whether in the form of a church or in the equivalent for whatever system of belief the human subscribes to, such as the stadium or iconography of said sports team. True Love seems to count as a form of True Faith in this situation as well. True Friendship can, as well, but the number of people capable of feeling friendship strongly enough to be a True Believer in that friendship is relatively small--most of those who have been capable of it have gone on to become the past Queens of the City of the Fireflies.

All of this only works when wielded by a True Believer. A cross, in and of itself, has no power against a fairy. Even a cross wielded by a christian doesn't necessarily have any power against a fairy. It takes a powerful act of faith for such things to work, and as such they are rarely used as a means of fighting fairies. A young couple in love doesn't necessarily count as True Love either, even if married or if the couple stays together forever. It takes a complete surrender of self in the support of one's belief or one's bond to power this faith.

An odd effect of Faith, Love, and Friendship that differs from other weaknesses to fairies is that it only affects fairies who work against the believer or the thing they believe in. Fairy magic cannot pierce a barrier of True Love to harm the lovers, but fairy magic used to support them, or to harm someone right beside them, is completely unfazed by the barriers. Likewise it can only be used as a weapon against fairies who stand in the way of the faith, love, or friendship of the attacker.

The reason for this has yet to be determined. Fairies believe it is effectively the form that Human Magic takes. If so, it does not resemble Fairy Magic at all--it is not possible to create "Spells" based on Faith. Things can be blessed or sanctified or hallowed, but this gives them no special properties other than being a conduit for True Believers to channel their faith in the future. Additionally, it doesn't appear to matter whether something actually was blessed as long as the Believer believes it was. A religious relic that proves to have been a fake is exactly as powerful as a real relic as long as the Believer sees it as real.