The Mountains of Madness expansion introduced a new action that can be performed by any investigator. As an action, an investigator on any space gains one Focus token. An investigator may spend one Focus token to reroll one die when resolving a test. There is no limit to the number of Focus tokens he can spend to reroll dice.
An investigator cannot have more than two Focus tokens. Focus can be often spent for other game effects stated in cards or other game components. They may not be spent in place of Clues.
Some effects, like the Reckoning effect of Ithaqua, require the player to spend focus or face an effect. Some items, like the Handcuffs, also require the player to spend focus.
William Yorick will start the game with a Focus.