There is a page devoted to the game of Thud! from Pratchett's Discworld, but it is not well maintained. They have the semi-official rules to play, but I am afraid they are incomplete and quite obscure.
Is there a better description of the rules, unambiguous and covering all situations?
A board game of Thud! was released several years ago. The rules are available online
On the dwarfs' turn, they may either move or hurl one dwarf:
Move: any one dwarf is moved like a chess queen, any number of squares in any orthogonal or diagonal direction, but not onto or through any other piece, whether Thudstone, dwarf, or troll;
or
Hurl: anywhere there is a straight (orthogonal or diagonal) line of adjacent dwarfs on the board, they may hurl the front dwarf in the direction continuing the line, as long as the space between the lead dwarf and the troll is less than the number of dwarfs in the line. This is different from a normal move in that the dwarf is permitted to land on a square containing a troll, in which case the troll is removed from the board and the dwarf takes his place. This may only be done if the endmost dwarf can land on a troll by moving in the direction of the line at most as many spaces as there are dwarfs in the line. Since a single dwarf is a line of one in any direction, a dwarf may always move one space to capture a troll on an immediately adjacent square.
On the trolls' turn, they may either move or shove one troll:
Move: one troll is moved like a chess king, one square in any orthogonal or diagonal direction onto an empty square. After the troll has been moved, only a single dwarf on the eight squares adjacent to the moved troll may optionally be immediately captured and removed from the board, at the troll player's discretion; or
Shove: anywhere there is a straight (orthogonal or diagonal) line of adjacent trolls on the board, they may shove the endmost troll in the direction continuing the line, up to as many spaces as there are trolls in the line. As in a normal move, the troll may not land on an occupied square, and any (all) dwarfs in the eight squares adjacent to its final position may immediately be captured. Trolls may only make a shove if by doing so they capture at least one dwarf.
The battle is over when both players agree that no more captures can be made by continuing to play, or when one player has no more valid moves to make. At this point the players count score: the dwarfs score 1 point for each surviving dwarf, and the trolls score 4 for each remaining troll, with the difference being the 'final' score. The players should then swap sides to play another round, and the sum of their final scores for the two battles determines the overall victor.
