The game begins with each player being dealt a "loyalty" card; it's possible (but not certain) that one player will be dealt the traitor card. With fewer players -- the game supports 3 to 7 -- there's a greater chance that there is no traitor. Shadows Over Camelot is a cooperative game, so the traitor wins if everyone else loses; otherwise, the players win collectively.
Points are represented by swords, which can be either white (good) or black (bad). On your turn, you must first do an action that will help the forces of evil as they try to overthrow Camelot, and then do an action which helps you or another knight complete a quest. Depending on the outcome of these actions, more swords may be put on the board -- either white or black. At the end of the game, the traitor wins if there are not more white swords than black.
"The only way to win the game is once you have at least 12 swords on the table, the majority of them are white," said Eric Hautemont, president of Days of Wonder. "And that doesn't happen that frequently."
Hautemont also said that the traitor element and the uncertainty that it brings to the game gives Shadows Over Camelot better replay value than some other cooperative games have.
During the game, you can accuse someone of being a traitor -- but only after at least six swords are already on the board, so you can't make accusations early on. If you're right, the traitor is weakened since he must then work from the shadows and two black swords are replaced with white swords. But if you wrongly accuse someone, one white sword is replaced with a black sword.
And if there's a traitor in the game but no one reveals him, two some of the white swords are replaced with black swords at the end of the game -- making it very difficult for the good guys to win.
Shadows Over Camelot was designed by Serge Laget and Bruno Cathala. It was my pick as the best board game published in 2005.