From the publisher:
The quest is on: Emperor Alexius has sent the Company of the Phoenix to rediscover the lost worlds of the Second Republic, to reach once more for the stars and welcome humanity's distant kin into the fold of imperial destiny. But these worlds have been separated for centuries and greatly differ from the Known Worlds. Each answered the challenges of the Fall and intervening years in unique ways. Some now pose ideological threats to the political and religious powers of the Known Worlds, or hide secrets that, once exposed, could radically tip the balance of power between starfaring empires.
Lost Worlds is a companion volume to the Star Crusade sourcebook, detailing the independent worlds of barbarian space -- each now up for grabs with the opening of the jumpweb. 16 planets are featured, along with new sentient alien races: the fungal Muazi, the silicate Ming and the energetic Z'go.