An organization spread across the world in several offices in every large city and hub of civilization. An organization that seeks to unite all sell-swords, all mercenaries, all assassination work and all assorted adventure work. If a village is being plagued by a dragon it's the guild who takes on the work to kill it. If a war needs soldiers it's the guild that supplies.
There are some rules for members of the guild:
1. Never work outside of the guild.
2. 20% of the pay for your work goes to the guild.
3. Never work against other guild members.
In exchange for this every member of the guild is guaranteed a bed to sleep in and food in any guild office, the opportunity to take guild approved jobs, as well as protection from unwarranted accusations like murder charges.
One can sign on for the guild for several time periods; you could only sign up for a single job, or you could sign on for life, Signing up is possible at any of the offices. While many could claim the title of h