Over countless eons of warfare, smart people — that is to say, the ones capable of passing on their genetic material to future generations — are the ones who found some place to be that is as far as possible from the on-going warfare.
That place was typically underground.
Eventually, these enterprising survivors started to connect this tunnel with that tunnel, and then those tunnels to these other tunnels, until eventually Ur was riddled by an interconnecting series of tunnels.
At the major intersections of these tunnels, Gnomes and Dwarfs built cities.
These cities developed economies and politics and all the things you’d expect of cities, until eventually it became the thing known as The Underdark.
At certain points, the Underdark nears the surface, allowing people to cross from the surface to the The Underdark. These are mostly at cities, crossroads, or other key strategic locations.
These points of egress were especially attractive to anyone fleeing law, debtors, in-law imposed nuptials, and other circumstances that might lead a person to say, “Feet don’t fail me now!”
And, herein lies both a problem and a solution.
The problem was: where you enter The Underdark is probably not the place you want to exit The Underdark; and, you never can tell what group of desperados might be controlling any given place you wish the exit.
The solution was: and, so it was decided that all of these roguish desperados needed to come to some sort of agreement, which was roughly speaking “[1] let’s all agree to rob, cheat, and steel from everyone else and [2] if I see you working my side of the street they’ll find you hanging by your entrails from the nearest tree.” It was simple, practical, and easy to implement.
This long-lasting and astonishingly durable solution has become the thieves’ guild known as “The Amalgamated Substrates of Ur.”
All rogues belong to The Amalgamated Substrates of Ur.
Proof of membership is a challenge and response in Thieves’ Cant. It doesn’t matter what the challenge or response is, since you either know Thieves’s Cant or you don’t. Any answer in Thieves’ Cant is a pass, and “say what?” is often the last words of anyone traversing from the surface to The Underdark or visa versa.
Ratticus Rex of The Amalgamated Substrates of Ur is The Bird Lady of Py-Ut, believed to lurking somewhere beneath Unpronounceable. The Ratticus Regent of Merderterre is Liquid Len of the Smash Bottle Men in Ur-et-Scatia. The Ratticus Regent of Schisseplatz is Squizling the Extremely Unlikable in Fortinbras.
(See also the note on The Amalgamated Substrates of Ur operations in Ur-et-Scatia, found on Page 4 of the Chapter 2 game notes.)