The DM is responsible for narrative flow, creating the scenario and setting in which the game takes place, maintaining the pace and providing dynamic feedback. In storyteller role, the DM is responsible for describing the events of the D&D game session and making rulings about game situations and effects based on the decisions made by the players. The DM can develop the adventure plot and setting in which these PCs participate or use a preexisting module. This is typically designed as a type of decision tree that is followed by the players, and a customized version can require several hours of preparation for each hour spent playing the game.
Your title here…The DM serves as the arbiter of the rules, both in teaching the rules to the players and in enforcing them. The rules provide game mechanics for resolving the outcome of events, including how the player’s characters interact with the game world. Although the rules exist to provide a balanced game environment, the DM is free to ignore the rules as needed. The DM can modify, remove, or create entirely new rules in order to fit the rules to the current campaign. This includes situations where the rules do not readily apply, making it necessary to improvise. An example would be if the PCs are attacked by a living statue. To destroy the enemy, one PC soaks the statue in water, while the second uses his cone of cold breath to freeze the water. At this point, he appeals to the DM, saying the water expands as it freezes and shatters the statue. The DM might allow it, or roll dice to decide. In the above example the probability roll might come up in favor of the players, and the enemy would be shattered. Conversely, rules do not fit all eventualities and may have unintended consequences. The DM must ultimately draw the line between creative utilization of resources (e.g. firing wooden arrows into a dragon, then using a spell that warps wood at a distance) and exploit (e.g. “horse bombing” – using a non-combat spell that creates a temporary mount, several dozen feet above an enemy; hiring several thousand commoners to form a line and use a rule that allows characters pass items to each other immediately to propel objects at railgun speeds.)