Dungeon Master's Guide 4th

Most games have a winner and a loser, but the Dungeons & Dragons Roleplaying Game is fundamentally a cooperative game. The Dungeon Master (DM) plays the roles of the antagonists in the adventure, but the DM isn’t playing against the player characters (PCs). Although the DM represents all the PCs’ opponents and adversaries—monsters, nonplayer characters (NPCs), traps, and the like—he or she doesn’t want the player characters to fail any more than the other players do. The players all cooperate to achieve success for their characters. The DM’s goal is to make success taste its sweetest by presenting challenges that are just hard enough that the other players have to work to overcome them, but not so hard that they leave all the characters dead. At the table, having fun is the most important goal—more important than the characters’ success in an adventure. It’s just as vital for everyone at the table to cooperate toward making the game fun for everyone as it is for the player characters to cooperate within the adventure.