Penultima is a game of inductive logic, played on a chess board.
Before the game starts, the Spectators decide between themselves which pieces they will write rules for.
When the secret rule for a piece is written, the Spectator also gives it a new name for the duration of the game.
These names, and the existence of any invoke commands, are announced to the players at the start of the game.
As in standard chess, the winning player is the one who forces his or her opponent's king (or equivalent piece) into checkmate.