"The City" is its common name. Each of the countless races, sub-races, ethnic groups, cultures and sub-cultures have their own name for it. Sometimes one of those names comes into vogue and The City has a new unofficial name for a few seasons. Mostly though, it's just called "The City".
Every Guild has their own private genesis story for The City. Their own private creation myth that puts them and their particular interests right there at the beginning of it all. Out in public though, no one wants to claim official ownership. No one wants to be responsible for the monstrosity The City has become. Actually being the rightful heirs to the place would be a lot of work. It's easier to just have a secret creation conspiracy. Those myths let The Guilds justify bleeding the place dry.
There used to be open War in The City. Everyone wanted something valuable that was there, smack-bang in the middle of it. Maybe it was beneath the ground, maybe it grew in some hidden grove, maybe it was locked away in a tower. Regardless everyone wanted that thing and there was War.
The Guilds fought for control of singular City blocks. Their war factories were practically built on top of one another. The Guilds could simply roll their death machines out the door and they would have cannon fodder to blast. This went on for centuries. What The Guilds were fighting for was lost to common knowledge.
Throughout these centuries The Guilds experimented with magic and machines, inventing new means of annihilation. Innumerable rifts were opened to the fortean multiverse, whole sections of The City were lost to space time mutation. The Guilds were not concerned about the Things that crawled out of these portals. In fact they quickly weaponized them. The Things were nothing compared to the horrors The Guilds were already inflicting on one another.
Finally The City was brought to the edge of utter destruction. It was brought to the edge of utter destruction over and over again. Once it actually flew over the edge of destruction (those space time mutations proved useful). Whether an external force finally decided enough was enough and stepped in, or The Guilds came to the conclusions that War was no longer in their best interest, the War stopped. The Law was introduced.
The Law is a curse that prevents any of The Guilds directly raising arms against one another.
The Guilds still hate each other though. And even though they have weapons in their arsenals that could implode whole galaxies they can't use them against each other. Instead their forced to use the scum that sit even lower than The Cities lowest underclass. That transient, gold-hungry class of outcasts the rest of The City looks down on. The Adventurers.
That's you. You're the deniable trash The Guilds use as their deadly puppets against one another. You are a Guild Dog.
(Guild Dog rules)