The Icons’ promised land, the vast cluster of star systems called the Third Horizon, is full of unsolved mysteries, ancient ruins, and deadly secrets. The mighty wonders left behind by the Portal Builders and the true nature of the mysterious Emissaries have only been myth to the common people of the Third Horizon – until now.
Some of the contents of this booklet: Description of the most important systems of the Third Horizon. A historical overview of the Third Horizon. The story of the Portal Builders and descriptions of the wonders they left behind. Tools and tables to help you create your own systems and planets. Mission generators for quick adventure creation. Expanded rules for space travel and unexpected encounters.
Another high quality supplement from Free League for Coriolis.
This takes the atlas in the main book and expands it to include more detail in some already overviewed areas, as well as provide new places to explore.
It also includes superficial details on some of the strange phenomenon in the Horizon. It also includes a lot of good tables to randomly create planets, encounters, ships, etc.
Reader beware: if you are a PC and not a GM, there are some spoilers about the metaplot in a part of the book, and might spoil some of the details in Mercy of the Icons.