Shadowscales are Argonians, born under the sign of The Shadow in Black Marsh. They are trained from childhood in the art of stealth and combat. Shadowscales serve their province as assassins. They have historically been used by the King of Argonia as a means to end conflicts and wars without major bloodshed, and to gather information on potential foes.
It has been implied by Veezara in the Dark Brotherhood Sanctuary near Falkreath, that the Shadowscales no longer exist. This has been hinted by his saying "I am the last of my kind, my order is extinct." This could be because of the events that occur in An Elder Scrolls Novel: The Infernal City, or more likely a side-effect of the decline of the Dark Brotherhood.
In Cicero's Journal, Volume I he talks about how the listener, Alisanne Dupre, and another Dark Brotherhood member named Rasha were deciding on whether to re-open a Shadowscale training facility in Black Marsh known as Archon, but they then abandoned the idea because they lacked the resources to do so.
Teinaava asks that the Hero eliminate a renegade shadowscale.
- In The Elder Scrolls V: Skyrim, Argonians may comment that the Dragonborn reminds them of "The Shadowscales of old," presumably if one acts in a stealthy manner or has a high skill level in the thief-related skill trees.