The Shadowscales were a sect of assassins in the service of the King of Black Marsh. Argonians born and bred in the arts of murder and stealth from their very hatching, they served the King of Black Marsh as secret killers until they came of age. At that time, according to ancient tradition, they were presented to the Dark Brotherhood as permanent new members.
In recent times, the tradition has virtually died out, leaving only one remaining Shadowscale, Veezara, in the service of an equally endangered Dark Brotherhood in southwestern Skyrim.