Dungeon Master

In the Dungeons & Dragons role-playing game, the Dungeon Master (abbreviated as DM) is a selected player who describes the game to the other players. In effect, the Dungeon Master creates the entire world and allows the players (the DM is usually not described as a player) to interact with that world.

The title was invented for the TSR Dungeons & Dragons RPG, and was introduced in the second supplement to the game rules (Blackmoor). To avoid infringement of TSR's copyrights, and to describe referees in role-playing genres other than sword 'n' sorcery, other gaming companies use more generic terms, like Game Master or Storyteller.

Role of Dungeon Master
The Dungeon Master prepares the game session for the players and the characters they play (known as player characters or PCs). The DM describes the events and decides on the outcomes of players' decisions. The Dungeon Master also keeps track of non-player characters (NPCs) and random encounters, as well as of the general state of the game world.

The game session (or "adventure") can be metaphorically described as a play, in which the players are the lead actors, and the DM provides the stage, the scenery, the basic plot on which the improvisational script is built, as well as all the bit parts and supporting characters.

DMs may choose to run a game based on a published game world, with the maps and history already in place; such game worlds often have pre-written adventures. Alternately, the DM may build their own world and script their own adventures.

DMs may run their game as frequently or infrequently as they wish; some gamers meet once a week or once a month, others only two or three times a year. A DM can easily run one-shot, unconnected adventures each time their gaming group convenes; in this case there is no connected plot, and the players can choose to play different characters in each session. However, a devoted Dungeon Master can string many such adventures into a campaign, in which the same heroes fight many different monsters and a few recurring villains, gaining treasure, reputation and power as they go. Such campaigns can last for years, even decades, earning a great deal of loyalty from their players, even as some players join or leave the game along the way.

A good Dungeon Master draws the players into the adventure, making it enjoyable for everyone. Good Dungeon Masters have quick minds, sharp wits, and rich imaginations. A Dungeon Master must also maintain game balance: hideously overpowered monsters or players are no fun.

Just as there are good DMs, bad DMs also exist. One of these is the rare but well-known type known as the "killer DM". This type of Dungeon Master enjoys killing the PCs, meaning that the imaginary character "dies" in the same way a character in a novel might -- they cannot go forward in the story, short of in-game mechanics like magical resurrection. The DM might get satisfaction out of creating monsters with very powerful game statistics, or designing fiendish traps that are virtually impossible for the characters to escape, but such a DM is likely to have trouble keeping players coming back for more adventures.

On the other end of the spectrum, there are DMs who try to fulfill all of the desires of their players by giving the PCs an easy time of acquiring experience and treasure. These easy DMs are often called "Monty Haul" DMs, a reference to the game show host Monty Hall.

Long-time role-players often joke about their experiences with bad DMs.

An example of a DM's duties, set in a fantasy universe
Days or weeks (or, in a worst case scenario, minutes) before a game session, the DM decides on the plot of the adventure which the players are to face.

Choosing a monster that will be tough but not deadly for the current power level of the characters played by the gamers in his or her group, she decides that the heroes are going to rescue a young prince kidnapped by an ogre. She makes a map of the ogre's lair, makes notes about the ogre's game statistics, and decides whether there are any other challenges (such as terrain or weather) that the party must face. He or shen then creates a memorable NPC, the prince's hideous and hysterical nanny.

On game day, the players gather around a table at the DM's house. The DM reminds the players of the game's setting and picks up the story where they left off, with the characters travelling on the road after their last adventure. She describes the woebegone nanny's appearance as she runs up to the PCs on the road and begs them to help save the prince.

As she knows that the PCs consider themselves to be good and noble heroes, the DM can expect them to agree to aid the nanny. Since the trail will be cold before they can return her to town, they must bring her along as they try to follow the tracks of the ogre. The DM asks them to use dice to test whether they succeed at using their tracking skill.

The DM also uses whatever acting abilities she possesses to "act out" the character of the nanny, wailing and fearful and clumsy, making sure that the heroes don't get the advantage of surprise. This also leads the PCs to interact with the NPC, "acting out" their own parts as they try to convince her to be quiet. This helps to create a deeper role-playing experience, where the player, instead of saying, "My character tells her to be quiet," or even "I tell her to be quiet," is led into role-playing with the GM:

Player (as Elf Hero): "Please, please ma'am, you must settle down, we don't want the monster to come after you too, do we?"

DM (as Nanny): "Oh, but my boy, my POOR BOY! That precious little boy, he's going to be EATEN UP!"

Another Player (as Half-Orc Hero): "Shut up, you old HAG!"

In a straightfoward adventure, the tracks lead to an abandoned watchtower, and the fighters in the party engage in combat with the waiting ogre -- again decided by dice-rolling supervised by the DM. A good DM will ensure that this part of the game is kept quick and lively, with decisive rulings, fast-paced game turns, and energetic descriptions of the ogre's actions and the results of the players' decisions.

Meanwhile, the nimble Rogue in the party climbs up the back of the tower, frees the prince from his ropes, and lowers him to the ground -- again, the DM determines how difficult these actions are and requires dice-based skill checks of some kind.

If she wanted a less straightforward plot, the DM might decide that there was no prince -- that the nanny was merely a human or shapechanged accomplice of the ogre, sent to lure unwary adventurers off the road so they could be robbed, killed, and eaten. In this case, the DM would be challenging the ability of the players to see through subterfuge and solve puzzles.

Either way, if the DM has chosen the level of difficulty well, the characters will have a good test of their abilities and wits. They will take a few injuries and be unsure of success, but with some good planning, teamwork, and bravery, will most likely overcome whatever obstacle the DM has placed in their path.

At the end of the session, the DM sometimes offers rewards: the characters may discover the ogre's treasure hoard in the tower. Based on how well they completed the adventure, the GM may give the players experience points, which can be used to improve the character before the next adventure, preparing them to face even tougher foes.