The 'Arduin Grimoire Trilogy' book Winds of Chance (Arduin Grimoire volume 8) from Emperor's Choice is an odd supplement in a way because it concentrates on some of the seedier sides of Dave Hargrave's Arduin rpg setting.
This is made clear within the Arduin books time & again. Because the thieves & asssasins guilds were clearly on the path to expanding their terrorities. And they were more then willing to do this through agents, thieves, and assasssins. This goes back into the the highest ranks of the royals a subject also covered in Winds of Change.