From the book: Blood Spawn: Creatures of Light and Shadow is a collection of monsters and adventures for the Birthright campaign setting. Far more than just a Monstrous Compendium volume, Blood Spawn is two accessories in one:
The creature listings offer more than a dozen new monsters designed specifically for the Birthright world. Some of the denizens haunt the Shadow World, others populate the waking world of Cerilia - and a few have infiltrated both realms.
The adventures provide a means of introducing these new creatures into your campaign. Each scenario features monsters from the book. By completing these scenarios, player characters have the opportunity to gain experience in the Shadow World and learn to penetrate its illusions.