From the introduction:
As the Dragon Master (DM) it is your responsibility to facilitate the game, describing the world to the players as their characters’ “five senses.” You determine the actions of non-‐player characters (NPCs), both man and monster; you also act as referee and rules interpreter. By creating adventures and allowing the player characters free reign to interact with the setting, you help to create magnificent stories of fantastic adventure. While Book 1 provides information every player should know, and Book 2 details the supernatural elements of the Five Ancient Kingdoms (5AK), it is this volume which provides the additional rules you need to run the game as the DM.
This is the third volume in a three volume set for the core rules.