Within the context of a roleplaying game, the game master (or GM) is the person describing the scenario and explaining to the players what the consequences of their actions are. They also play the role of every NPC that the players will encounter on their adventure. Typically, the GM also writes the campaign as well.
For The Game of Rassilon, Ben Paddon serves as GM. He primarily runs the sessions, though co-writes the campaign with Michael Nixon. Paddon has also asked Nixon's input for some rule decisions and narrative choices during episodes.
Nixon briefly served as GM for the dream portions of Lethargica.