Dungeon Master's Guide.. Core rulebook II. VG condition. This starts our with what it means to be a Dungeon Master and how to do things such as run a game session, change a rule, or work with the players to make sure everyone has fun. The information in this chapter isn’t the sort that you’ll refer to during a game, but you’ll want to read and think about the issues discussed here well before you ever sit down to run your first game.