Fairies in folklore can't touch or are harmed by iron, especially "cold iron," which repels, burns, or binds them, making it a primary protective element for humans. Other things fairies supposedly dislike or can't cross include running water, the sound of church bells, and sometimes salt, bread, or true names, though iron is the most famous deterrent.
In the Disney film Maleficent, the title character reveals early on that iron is lethal to fairies, and that the metal burns them on contact.
People forgot what cold iron meant and why it was effective, and started to believe that iron itself had some sort of magical, anti-faerie property. And because of the way magic works, that myth became truth. Long story short, the Fae are vulnerable to iron because we believe they are.
Also, fae blood is toxic to vampires, an embodiment of the dark, in many tales as well. Fairies are weak to steel & poison: In most folklore, fairies have an aversion to iron (be it a weakness or straight killing, most describe it as poisonous).
Further support for this idea is that fairies were thought to be afraid of iron and could not touch it. Living outside of society, the hiding people probably stole food and attacked villages. This might explain why fairies were often described as playing tricks on humans.
Reportedly, “[promises] are very sacred to the fae and hold a lot of power so if you break a promise, expect there to be some angry faerie cursing you.” Never eat food that a fairy offers.
Fairy wings tend to soak up water and become weighted down. On land, this prevents fairies from flying; in water, it can drown them. Even water-talent fairies cannot swim. The Wing-washing-talent fairies wash the fairies wings to clean them when they are dirty.
Two of the common weaknesses of Fairies in folk tales are salt and iron : salt has power and can prevent or diminish Fey magic, and iron is harming to the touch to Fey. They dislike it so much that hanging a horseshoe on your door is supposed to prevent them from entering.
In most folklore iron is the only metal that can cause human-like injuries to a faery and usually a part cut off with iron cannot be restored. Iron chains can be used to restrain a faery or restrict their magick.
Fairy tale fey creatures are often vulnerable in folklore and some fantasy literature to the touch of iron and silver, for example in the novel Three Hearts and Three Lions by Poul Anderson where the elves make due by using magically produced aluminium instead of the two metals.
It's quite well-known that, amongst the varied substances to which fairies object, everyday, ordinary salt is one of the most repellent for them.
Never, ever accept a gift from a Fey.
And never, ever verbally say thank you for a gift. It implies that the receiver owes them something, like a food, your firstborn, or a favor (no matter how deranged).
One superstition is that anyone who steps into an empty fairy ring will die at a young age.
Salt was put to use as a potent protection against fairies, although some say it had to be blessed in the same manner as holy water to have any effect, or that fairies would be forced to stop and count every grain before they could pass, and so it was sprinkled across thresholds and windows.
Never thank them: In some traditions, saying "thank you" to the fae can be seen as an acknowledgment that you owe them a favor in return, which may not be something you want. Avoid using their names: Some believe that speaking a fairy's true name gives you power over them.
If a sharpened scythe was placed edge-up in a chimney it would repel fairies, and an iron bolt or lock on a door would guard a house, while an axe placed under the pillow would protect the sleeper and striking a fairy with iron will result in its instant disappearance.
In the old stories this phrasing only meant that you accidentally revealed your full name to the Folk and thereby gave them power over you. With your name they could enchant or lure you, they might even be able to influence your behavior. When you know someone's name you have a certain measure of power over them.
They are known for the following: Fairy Kiss: Fairies have the ability to mark people with a sort of invisible protection charm by kissing them on the nose. It prevents the Fairy Queen from killing them (at least not right away) as the Fairies show trust in whoever they kiss and possibly would not want them killed.
Fairies have a major weakness: iron, which repels them and dilutes their supernatural powers. Amulets made of iron keep fairies away.
Leave offerings: Fairies are said to appreciate gifts and offerings. Leave small trinkets like crystals, shells, or pretty stones around your home or garden as a sign of goodwill. Create a fairy circle: A fairy circle is a ring of mushrooms that is said to be a portal to the fairy world.
Circles of mushrooms known as fairy rings were described as being left behind by fairy footfalls after a night of dancing under the moon. It is considered very bad luck to break a fairy ring, causing seven years of bad luck to fall upon anyone who damages them.
Fairy is weak to Poison. Fairy is Not Very Effective against Poison. Because Steel. Fairy is Not Very Effective against Steel.
The smell that was so noticeable and inexplicable might have been a clear sign of Hyndman's contact with the faeries. That would unquestionably have been the interpretation placed on matters on the Isle of Man, where the smell of fairies was a well-known phenomenon, and was said to be sour and strong.
In ancient Ireland, the offerings left out for our fae friends were usually a bowl of milk or freshly churned butter. As time moved on, the garden fairies began to experience growth in eating sweets and cakes, because of their plentiful and boundless appetites.
Faerie Etiquette: What to do if you meet one of the Fair Folk