The Arcane Brotherhood

Gods help anyone who kills an Arcane Brother, purposely or otherwise. It’s the oldest rule of their guild, a rule without exceptions: kill a Brother, and the whole guild drops whatever it’s doing to come after you. They seek you out by any means they need to use. They kill your friends, your family, your associates. They burn your home. They destroy everything you’ve ever built. Before they finally let you die, they make sure you know that your line has been wiped from the earth, root and branch. - Mirt the Moneylender

The Arcane Brotherhood is a society of mages that operates, more or less openly, out of the city of Luskan. While the City of Sails is ostensibly ruled by the High Captains, all former pirate lords, the Brotherhood is the true power behind the city. The arch-wizards who rule the Brotherhood have their fingers in any number of unsavory pies, including piracy, slavery, drug trafficking, and smuggling, in addition to their own legitimate trading ventures. The Brotherhood undertakes these activities as a means to an end: establishing economic and political sovereignty over the North.