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.
Alven
Fairy-like gardeners.
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 were the original Arak variants from "The Shadow Rift", their Van Richten's Guide writeup made it clear that these are only the most common and prominent forms that an Arak can take. As such, two other breeds of less-common Arak are also known to exist in 3e, and their VRG sourcebook also contains rules for creating your own Shadow Fey from scratch.
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.