In Barsaive, magic, like many things in nature, goes through cycles. As the magic level rises, it allows alien creatures called Horrors to cross from their distant, otherworldly dimension into our own. The Horrors come in an almost infinite variety—from simple eating machines that devour all they encounter, to incredibly intelligent and cunning foes that feed off the negative emotions they inspire in their prey.
In the distant past of Earthdawn's setting, an elf scholar discovered that the time of the Horrors was approaching, and founded the Eternal Library in order to discover a way to defeat them — or at the very least, survive them. The community that grew up around the library developed wards and protections against the Horrors, which they traded to other lands and eventually became the powerful Theran Empire, an extremely magically advanced civilization and the main antagonist of the Earthdawn setting.
The peoples of the world built kaers, underground towns and cities, which they sealed with the Theran wards to wait out the time of the Horrors, which was called the Scourge. Theran wizards and politicians warned many of the outlying nations around Thera of the coming of the Horrors, offering the protection of the kaers to those who would pledge their loyalty to the Empire. Most of these nations agreed at first though some became unwilling to fulfill their end of the bargain after the end of the Scourge, wanting to have nothing to do with the bureaucratic nation run on political conflict and powered by slavery. After four hundred years of hiding, the Scourge ended, and the people emerged to a world changed by the Horrors. The player characters explore this new world, discovering lost secrets of the past, and fighting Horrors that remain.