I was hoping some of you lore buffs could help me to understand what the 'Shadowscales' are. Thanks.

If an Argonian is born under the sign of the Shadow, they are given to the Dark Brotherhood for training (there was a training facility in Archon). They are then handed back to Black Marsh to serve as the "King of Black Marsh"'s assassins, although some Shadowscales stay within the Brotherhood. We don't know much more than that.

Argonians born under the sign of the shadow, raised and trained by the Dark Brotherhood to become assassins at later times. Used primarily by Argonian leaders and the Dark brotherhood.

The Shadowscales died out a few years ago when the Dark brotherhood fell apart.