From the back of the box:
Far off the traveled paths of wildspace, a lone crystal sphere bobs in the phlogiston, waiting for unwary visitors greedy for adventure. This is the Astromundi Cluster, a setting easy to find... and nigh-impossible to escape.
Unique among the spheres, Clusterspace holds no planets - only thousands of asteroids that wander in a seemingly endless realm. Here Arcane, neogi, illithids and humans have learned to coexist in a semblance of peace to ensure their survival. But the carefully-wrought balance of power may change at any time, and heroes have to choose their allies and enemies carefully. Within the Cluster, there are no such fine guarantees as honor or trust between friends and foes.
The Astromundi Cluster accessory includes:
A 32 page player's manual to the factions and features of Clusterspace. The Astrogator's Guide gives players all the information they need about the races and major asteroid clusters of the Shattered Sphere, life and culture in the Cluster, and new ship and weapon types.
A 64-page DM guide, Adventures in the Shattered Sphere, which reveals to the Dungeon Master the secrets of the Astromundi Cluster, from the magically powerful Antilan Empire to the behind-the-scenes plots of the Arcane.
A 96-page DM book describing in full detail the most important asteroids and other features of teh Cluster, its physical makeup and economy, and new magic and spells.
Two full-size maps illustrating the planetary orbits of the Cluster's celestial bodies, and schematic layouts of a feared crystal citadel and important trading post.
Twenty-four cards filled with ship blueprints, important faction illustrations and descriptions, orbital maps, and more.