Grim Dark Dungeon Master introduces the concept of competitive dungeon building.
In this game one player assumes the role of the Grim Dark Dungeon Master who wants to protect key areas of the dungeon to prevent interruption to the dark doings occurring there.
Meanwhile 1 to 3 other players assume the roles of heroes, working together to penetrate the dungeon, push past obstacles, and find three key locations to purge of evil and slow, if not outright stop, the spread of darkness across the land.
However, neither the Grim Dark Dungeon Master player nor the hero players know the layout of the dungeon at the start of the game. That will be determined as the game is played with both sides trying to influence the position of key rooms as they are played on the table.
Thus, both sides compete to build the dungeon that will most easily achieve their goals.
—description from the designer