Nightmare Cathedral prominently contains the haunting artwork by the late polish surrealist painter and sculptor Zdzisław Beksiński.

Nightmare Cathedral is a strategy game for up to 4 players, where victory is gained by controlling the map, managing resources, following objectives and selecting when to engage in fast and furious conflicts to gain strategic advantage. Over the course of the game, you will perform actions — and follow the actions of other players — to create units, build fortifications, maneuver around the map, and spend your followers to climb the ritual track. Throughout the game, a haunting Cathedral will grow in the center of the board, and once it is finished, you will face — and gain the ability to control! — the physical manifestations of nightmares that are released upon the world. Nightmare Cathedral is a game of both strategy and tactics, where finding smart ways to follow your objectives and combining different strategies is the ultimate route to victory.

Every player has a set of Action cards. These Action cards display various effects that can be activated either on that player's own turn, or as a reaction when other players perform that action. When you're the Active Player, you will move your Dreamer to a Day space that is not adjacent to its current Night space and perform the Action associated with that Day space, as outlined on your own Action card.

Inactive Players might react to your chosen Day Action.

The game is split into two stages. During the first stage, the titular Nightmare Cathedral is constructed, piece by piece, as the game progresses. Once the last Cathedral piece is placed on the main board, the Nightmare Cathedral is finished, at which point the game transitions to the second stage. From this moment on, Nightmares, powerful beings channeling the presence of the finished Cathedral, enter the game board, Devouring Units in their path. However, as a player, you can also take actions to control a Nightmare, Commanding it to do your bidding.

The game end is triggered by one of the following ways:

• The Time marker reaches a certain space on the Time track.
• A single player gets to the final step of the Ritual track.
• A specific number of Units have been devoured.