Willpower is a character attribute, designed to reflect the psychological state of the player, in the Fighting Fantasy gamebook Beneath Nightmare Castle. At the start of your adventure, your initial willpower is calculated by rolling 1d6 and adding 6 (i.e. 7-12) - the higher the score, the more psychologically resilient You are.
Willpower is tested in the same manner as luck - when asked to Test your Willpower, you must roll 2d6. If the result is less than or equal to your current willpower, you have been successful in coping with your current situation. If you roll more than your current willpower, you have failed to cope, psychologically, with your current situation and will suffer as a result. Whether the result of your test is successful or unsuccessful, you must deduct 1 point from your current willpower.
There is an acute complication, however. If you fail a Test your Willpower roll, and your current willpower is below 6, you will lose your grip on your sanity and your adventure will end.