From the back cover:
Welcome to Rokugan, a land of breathtaking beauty, where eight clans of powerful samurai warriors struggle against one another in a constant war for supremacy. Rokugan is a land where powerful shugenja and enlightened monks command the forces of the elements. It is a land where ancient races of naga, ratlings, dragons, and other mythical creatures live and walk beside mortal men. It is a land where cunning assassins stalk the shadows, walking the dark path of the ninja. It is a land where honor is as sharp as steel.
This book contains all the details you need to play Dungeons & Dragons adventures in the Legend of the Five Rings setting.
To use this companion, you must have Oriental Adventures.