Myrk
Several centuries ago, the world was mysteriously shrouded in myrk: a harmful, uninhabitable, gaseous substance. In the passing decades, settlements crowded together and have largely found ways to keep myrk at bay, but the majority of the land is still beyond civilization’s touch.
The few cities of the world are connected by myrk-warded routes. Villages outside of this network may relay on myrk-resistant gear or guides to travel, but only the truly wealthy can travel freely. Danger lies in the myrk, both from the gas itself and in the monsters that thrive in it. As adventurers, you have some degree of myrk resistance— but sustained exposure is deadly, even for you.
There is much more of the world to discover, both in ways unknown and already-known to your character. Questions always lead the way to answers.