Some Discworld scholars believe that Stealth Chess is the original form of chess in their world; this belief is corroborated by the in-world discovery, in a tomb in Muntab, of a preserved corpse with an 8x10 board embedded in its skull and a pawn hammered up each nostril.
Differences from Chess
While mostly similar to regular chess, an extra piece has been added to the game, and the board widened to accommodate it. The black and white board gains an extra row along the left and right sides, making it 8 squares long by 10 squares wide; these extra rows are red and white in colour, and named the Slurks. These new squares can only be used by the new 'Assassin' piece, which starts the game on either side of the Rooks.
The Slurks are basically a second, "invisible" board. Many players consider the assassin to be moving "underneath" the actual board, ready to pop out when it has reached their intended destination.
The Assassin can move one square in any direction, or two squares to capture a piece. The Assassin can enter or exit the Slurks at any time, but there is an advantage to staying hidden. On exiting the Slurks, the assassin may make as many moves as it has taken within the slurks and, optionally, a capture move.
The Assassin may take pieces of its own colour, but may not take opposing Assassins.
If an Assassin enters the Slurks and takes five moves within them (in any direction, including back and forth), it may then appear in any square that is five moves from its original entry point into the Slurk. It is then still able to make a one-square move to capture.
This makes the Assassin a very powerful piece, and a very effective counter to certain strategies depending on specific pieces. It can quickly win the game if one manages to take control of the Slurks and access the King directly. However, 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.