Shadow Fey
The Shadow Fey (or Arak, as they call themselves) are a species of darkness-loving, shadow-touched Fey native to the Demiplane of Dread, where they inhabit the dark faerieland domain known as the Shadow Rift. Originally slaves to a terrible demon-god from the Plane of Shadow known as Gwydion, a great hero of their people managed to lead them to freedom by finding a passage out of that dark realm and into the Demiplane of Dread. Which is the first time in history when getting stuck in Ravenloft was an actual improvement for anyone's lot in life.
Shadow Fey are detailed in the AD&D sourcebook "The Shadow Rift" and in the 3.5 sourcebook "Van Richten's Guide to the Shadow Fey".
A Diffuse Race
As with the normal Fey, the Arak come in many different shapes and sizes, each with its own particular focus and its own role to play in the greater Arak society. But, at the same time, the Arak are still one race at core; not only do they readily interbreed, but infant shadow fey resemble no specific species - they transform into a distinctive breed when they reach maturity, and may not take the same breed of either of their parents. The "Nine Breeds" listed here are simply the most common manifestations of Arak adulthood, and there are literally countless others; Van Richten's Guide to the Shadow Fey even contains specific rules for creating new kinds of Arak.
Alven
Commonly known as "Carrot Tops" or "Fire Flits", alven are the closest thing amongst the arak to the standard Fairy, appearing as ten to fourteen inch-tall butterfly-winged elves with green skin and bright orange hair. These "shadow pixies" are the Arak's gardeners and horticulturalists; they love plants of all kinds, but especially favor flowering plants, and tend to the gardens and groves of the Shadow Rift. When they deign to visit the rest of the Demiplane of Dread, they gravitate towards gardens and fields of wildflowers, particularly night-blooming varities. They can even shapeshift into the forms of insects to better blend into their surroundings; butterflies and bees in AD&D, but also wasps, dragonflies and moths in 3rd edition.
It goes without saying, but alven are fiercely protective of their gardens; picking the flowers or damaging the plants will incite their wrath, and they are surprisingly fierce fighters; by flitting about in mid-air, alven can generate magical effects through their "dance", which requires the victim to make two saves and causes them to be enthralled if they fail the first, or blinded & deafened if they fail the second, and also possess a number of druidic spells (cast spells of the Plant Sphere as if 5th level Clerics in AD&D, spell-like abilities of Barkskin 3/day, Disguise Self 3/day, Entangle 3/day, Hallucinatory Terrain 3/day, Plant Growth 1/day with a caster level of 5 in 3e). They're also immune to attacks with stone weaponry and to electricity, and they can see through invisibility in both editions, although it requires a Survival check for them to do so in 3e. To get on one's good side, try complimenting its garden or present it with gifts of seeds or living plants.
Alven have a mystical ability to communicate with plants, which makes it impossible for them to lose their way in their preferred surroundings (basically, they can Find The Path at will when amongst living vegetation). This may be why they regard cut flowers as an obscenity; presenting an alven with a picked flower will guarantee that it attacks you.
When an alven makes a Changeling, they invariably choose somebody who will be useful to them as a gardener, although sometimes that is just in the role of brute laborer doing things that the alven is too small and weak to do.
Brag
Hard-working, hard-playing builders.
Fir
Clever little artificers who love to tinker with machinery.
Muryan
Battle-crazed berserkers.
Portune
Dedicated healers.
Powrie
Spiteful little pixie-redcap hybrids.
Shee
The most classically elf-like breed.
Sith
Macabre neo-elven necromancers.
Teg
Feral and savage predators.
The Lesser Breeds
Whilst the nine breeds listed above the most common and prominent forms that an Arak can take, other species do exist. However, only two varities of them have ever been officially statted.
Waffs, or "Shadow Dryads", appear in the Shadow Fey VRG sourcebook. These ephemeral, malevolent shadow fey are essentially sapient clouds of vampiric mist, hiding from danger within a host-tree and pouring forth to drain the life from whatever catches their eyes.
Gwytune appear in the Ravenloft Gazetteer V. Extremely rare, these court-neutral Arak care only about studying and mastering arcane magic, making them a breed of wizards amongst a race that normally is happy to be sorcerers. They appear as wizened and spindly elf-like beings with long, dexterous fingers, faintly lilac-tinged skin, auburn hair, golden goat-like eyes, and curling ram's horns. Able to shapeshift into the form of sheep and goats, gwytune are physically inoffensive, but powerful casters, and like certain other breeds of Arak, have a special connection with fate that makes them much harder to hurt than you'd expect.
The Courts
The Arak divide themselves into two Courts, each headed by one of the children of their slain hero-king, who helped free them by tricking Gwydion and then sacrificed himself to hold the line against the umbral demon-god's pursuit.
The Seelie Court
Sworn to Maeve, mostly containing shee, portune, alven and fir.
The Unseelie Court
Sworn to Loht, predominantly sith, muryan, powrie and teg.
Relationships with Mortals
In typical Gothic fairytale fashion, the Arak don't exactly see mortals in a very positive light. The most typical way to describe it is that the Arak view mortals as fascinating, wonderful toys; the Seelie are those shadow fey who play nicely with their toys, whilst the Unseelie are those who enjoy breaking them.
The Arak can magically separate victims from their souls by stealing their shadows, a ritual they can use to turn the stolen soul into a magical drone they call a Changeling.