Haven is a small town on the coat of Massachusetts, not far from Boston and an easy commuting distance from Brown University. To most of the world that is all it is, and even to most of the residents its not so different. They have a few extra rules they live by, like don't venture out into the forest alone and never go outside when its misty no matter what you hear. But apart from these they live much as anybody else does.
For others though, for the members of the supernatural world Haven is something entirely different. It is a place where one can cross between the worlds, a way-station and the last one of its kind on earth. This makes it a place of great import, where supernatural groups can use these connections to gather valuable and powerful materials and goods from those other planes. It is also where any threats from outside earth would first arrive, making it an important fortress and beachhead should earth ever be attacked.
For these reasons it drew the interest of the three most powerful supernatural organizations in the world, The Hand, The Order and The Temple. These organizations all share a common goal in holding Haven as a defensive bastion, but also are all in competition over the resources that can be obtained from it. Many attempts to strike an accord between them failed, until finally they settled on a less diplomatic solution. The three factions came together to work a great magic over the town, called Sanctuary this magic protects anybody inside the town from serious harm by others. And this protection is where the towns name derives, although not all are equally protected and several ways to bypass the protection have since been discovered. The spell allowed the factions to compete and conflict with each other, without ever being at risk of weakening the forces they have set to guard the earth from intrusion.
Others also found the town of interest, supernatural scholars and magical practitioners came to experiment with the supernatural power to be found there. Others came to make use of sanctuary to hold conferences or shady meetings with those they couldn't otherwise trust. And the town drew the attention of a criminal supernatural element as well. Throughout most of the world one of the three factions would govern the supernatural and enforce laws they chose, but in Haven if you got the protection of Sanctuary, nobody could enforce a law against you, freeing up certain types of criminals to do largely as they wished as long as they didn't need to kill anyone. Supernatural tourists also became somewhat common.
When supernaturals fight, they usually do so in a shadow world, a darker mimicry of our own called the shroud. There they can conflict freely, without ever being seen or noticed by the normal members of humanity. It is also where threats to earth will generally arrive. It usually takes them a few hours or even days to make the transition fully into earth proper.
A manipulation of the shroud called pathing, allows some supernaturals and those they take with them to travel vast distances very quickly. Due to the latent power of Haven pathing to and from it tends to be a fair bit faster than elsewhere, and factions often deploy operatives there for just that reason, wanting to take advantage of their ability to quickly deploy to missions or conflicts around the globe.
As a result of all these disparate groups and individuals Haven has come to resemble something akin to a frontier town from the old wild west for the supernatural community, with its fair share of prospectors, outlaws and soldiers.