From the introduction:
Thousands of years ago, a slumbering assortment of human refugees was brought to a lifeless frozen planet by an enormous space ark. Clouds of terraforming nanotech were seeded into the planet’s weak atmosphere. Over the course of a few decades, the frozen world grew warm and vibrant with plant and animal life, just like the distant Earth that humanity had left behind. The ark’s advanced A.I.—after overseeing the terraforming process—woke its human cargo and sent them down to the planet’s surface, beginning the story of humanity anew on an alien world both familiar and strange.
Rather than living in unity, humans soon fragmented into a number of warring kingdoms, the majority of which returned to a feudal existence based on technologies they could build with their hands: agriculture, masonry, metalworking, and so on. The nanotech clouds, space ark, orbital A.I., and other unseen wonders became the stuff of myth and legend. Only a few small enclaves—hidden on mountain peaks and in secret valleys—maintained expertise in advanced technology, allowing a select group of people known as the Lumen to live extremely long lives thanks to seemingly magical devices.