It is similar to normal chess, with the exception of an extra piece and the widening of the board by two specially coloured (red and white has been reported) fields on either side, known as the Slurks. The extra piece is the Assassin (appearing on either side of the Rooks in the beginning of the game), the only piece to be able to move in the Slurk.
The Assassin moves one square in any direction, and two to capture; however, on exiting the Slurks, the assassin may make as many moves as it has taken within the Slurks and, optionally, a capture move. An example may clarify: If an Assassin enters the Slurk and takes five moves within the Slurk, it may then appear in any square that is five moves from its entry point into the Slurk, before making a one-square move to capture. The Assassin may take pieces of its own colour, should this give the player an advantage, but may not take the opposing Assassins.
This makes it a highly powerful piece and a very effective counter to certain strategies depending on specific pieces, and can quickly win one the game if one manages to take control of the Slurk and access the King directly. The Discworld Companion notes, however, that players should also take care that they don't focus on the opponent's Assassin exclusively to the point that they lose track of what the opponent's other pieces are doing.
According to the Companion, some Discworld scholars believe that Stealth Chess is the original form of chess in their world; this belief is corroborated by the discovery, in a tomb in Muntab, of a preserved corpse with an 8×10 board embedded in its skull and a pawn hammered up each nostril.