From publisher blurb:
The Scarred Lands Player’s Guide mentions a number of criminal organizations, thieves’ guilds and factions with influence across the face of Ghelspad. The most notorious and widespread among these factions is a group known as The Scaled. In the Rogue class description, members of the Scaled are listed as potential spellcasters. The descriptions of the Filch and Flash spells in the Scarred Lands Player’s Guide state that these spells were invented by members of the Scaled.
However, there is no Rogue Archetype provided in the Scarred Lands Player’s Guide with spellcasting ability. The Master of the Scaled is presented to allow players a means of exploring the arcane thieves of one of the most well-known and romanticized criminal organizations in Ghelspad.