Willpower represents long-term mental fatigue. Decreasing as mental abilities are used, it will increase naturally at a rate determined by one's race and the level of mini-skill called Philosophy.
The act of meditation will enable an adventurer to regain lost willpower. Adventurers who use more than the usual amount of willpower might find it useful to invest in a megalith tattoo or the artefact known as the Circlet of Will.