The Dungeon Master client (DM client) is a special mode of operation of the Neverwinter Nights client that is used by Dungeon Masters to enhance the game for players.
It is started by selecting "DM client" from the opening (splash) menu, or by running the client executable (nwmain.exe under Windows) with the command line argument -dmc. (Under Linux, a shell script called dmclient is provided that invokes the executable with this argument.)
The DM client is restricted to multiplayer games, and most servers require a password for anyone attempting to connect with the DM client (i.e. as a Dungeon Master).
Recognizing the inherent difficultly of being a good DM, BioWare sought to ease the job through the designing of the DM client. They felt that the Dungeon Master is a very special person, someone to be catered to. They wanted to allow DMs to focus on telling stories, rather than on trying to navigate a clumsy interface or learn a poorly documented system. For familiarity and ease of use, the Dungeon Master interface was designed as an extension of the player interface. This means that players familiar with the regular client will be familiar with the general interface of the DM client even though the enhanced functionality will be new.
Among the extended capabilities of the DM client are the ability to create objects, possess and control any monster or non-player character (with or without DM powers), award experience points and gold, and generally control the world. The character of a player using the DM client (a DM avatar) cannot be injured or killed and can only be seen by regular players if the DM desires to be seen.