The Brotherhood Source Book

From the introduction:

The Light emerged when the Darkness began its encroachment. Earth was the past, mankind’s birth-world, abandoned centuries before. Humans had spread across the inner worlds of the solar system and beyond, and humanity had experienced an unprecedented golden age of expansion and prosperity. However, with triumph comes hubris, and with hubris came the fall. All that mankind had built was on the verge of collapse, and decades of decay and war had undone centuries of progress.