From the Introduction:
The Eleven Pillars is a ‘new’ setting in an ‘old’ world. This is a setting in which the civilized folk have moved up to the Mountainlands (the tops of mountains above the cloudline) because the Lowerlands have been over-run with all sorts of vile creatures for centuries. This mass movement of civilized people is referred to as the Great Migration. The cities and settlements in the Lowerlands have fallen to ruin. Trouble is brewing, however, and it is a time of exploration and expansion for the civilized populace of the Mountainlands. Many are pushing for re-settlement of the Lowerlands while others maintain that it is too dangerous and unlivable below the cloudline.
The Eleven Pillars is a setting guide suitable for use with any fantasy RPG.