The Fey

Listen here child,

and hear me well

a warning to you,

of the Fey I'll tell

Give them your name,

they'll take your tongue

they'll shatter your life

and steal your young

So if in the forest

you spy a ring

run fast, run far

or they will sing

The songs they sing

are joyful a rhyme

that once you've heard

you'll lose all your time

So listen here child,

and hear me well

beware the Fey,

they'll drag you to hell

- A children's tale heard in Mournhallow, southern Middenheim

The fey: a primordial, chaotic collection of beings incomprehensible to mortals. No race on this world is older than the fey, not even the Eldar. The traces of the fey on this plane are few now, for over the long millennia they have returned to the realm of the Feywild, a place of wild, frenzied life. Yet traces remain: standing stones on lone hills, circles of redcaps in glades, and places where no mortal would dare venture, where children have ventured never to return. A few fey still make their home on the Material plane, and more still venture forth for obscure reasons, giving rise to local deities, folk myths, and wild rumours.

The multitudes of Fey are bewildering to mortals, as no two seem to look alike. They are as diverse as nature itself, with dozens if not hundreds of subtypes known. Their only common bonds are their division into the Unseelie and Seelie courts and their respectful adherence to the whims of nature. Though Fey can seem capricious, few of them are truly evil. Instead, they serve the mysterious machinations of the Courts, divulging their true goals to no one else. It is this enigmatic nature, and their affinity for magic, that have made the Fey objects of both worship and revulsion across the realms of the Material Plane.

The Two Great Courts:

Fey society is split into two broad groups, both as chaotic and unpredictable as nature itself, each both in opposition and deep union with the other. Once, the courts would alternate in predominance, but the Unseelie court has long fallen out of favour.
The Seelie Court, ruled by the demigod Archfey Queen Titania, is a place of gaietious greenery, a forested tree-city of vibrant colours, a land bathed in an eternal summer. Its chief denizens are pixies, satyrs, redcaps, and talking animals. Despite its beauty, the Seelie Court is an exceedingly dangerous realm: a place where one may be cursed for uttering their own name, dance for an entire week, or sleep for a decade. The name "Seelie" literally derives from "the Seen", and its culture completely reflects this. Its denizens are outgoing, charismatic, beautiful, and self-absorbed.
The Unseelie Court was once ruled by King Oberon. In the aftermath of his mysterious demise, several claimants have risen to claim the Briarthrone, and the Unseelie domain has fallen into disrepair, its denizens scattered. Its followers include relicts, bumbakvetches, boggarts, and other creatures of winter. Unseelie fey are literally "unseen" - that is, they are the ones who hide in shadows, watching and learning about the world. They tend to be empathetic, shy, and curious. As the Unseelie court places little value on physical beauty, many of its members are unappealing, even downright hideous.

Fairies
The most humanoid of the fey, fairies are clever, long-lived creatures from which many of the intelligent races of the world descend. A fairly could almost be mistaken for a small elf, were it not for their insectile wings, small size, and uncanny, animal-like mannerisms. Though there are no rules in the Feywild, fairies tend to be the scholars, diplomats, and courtiers of the Courts.
Pixies
Pixies are an even smaller subspecies of fairies. Small enough to stand in the hand of a mortal, Pixies are mischievous, cunning, and exceedingly self absorbed members of the Seelie Court. They delight in playing practical jokes.
Atheracai
Atheracai, or Changelings, are the most powerful and oldest Fey known to exist. Possessing mutable form, they can take the appearance of any being they choose. But each changing takes a toll, and the more a changeling changes, the less likely they are to be able to undertake another changing. For this reason, changelings are now exceedingly rare.
High Fey
High Fey are descended from Changelings who lost the ability to shapeshift. Now trapped in a more or less humanoid form, they rely on illusion magic to disguise themselves. 
Archfey
An Archfey is the term for a Fey who has attained the status of a god.
Relicts
The broad term Relict simply specifies a creature that was once of the Fey, but has since been separated from the Feywild permanently. A relict continues to maintain a Fey appearance and mannerisms, but considers the Material plane its home. Examples of relicts include the Ladies Three, leshys, and shaelmaars.
Boggarts
Boggarts are a misunderstood species of Fey. Thought responsible for stealing children, spoiling milk, and a variety of other unpleasantness, boggarts are merely uncouth, hideous servants taking the appearance of a bulbous, vaguely humanoid plum. Despite their outward appearance, the Unseelie boggarts are known as extremely dutiful and loyal servants, if not the most intelligent of creatures.
Redcaps
Said to be created as a joke by Queen Titania long ago, redcaps are evil, bloodthirsty creatures who appear to be little more than oversized garden gnomes. Redcaps have a constant, insatiable lust for killing and are known for screaming obscenities at passers by before violently butchering them. Technically Seelie, redcaps are used by both Courts to spread chaos and confusion. Redcaps grow from spilled blood, where they initially take the form of red pointed mushrooms sticking out of the ground. Woe betide anyone who tries to pick one of these mushrooms.