Spells are magical rituals that an investigator can perform using the Lore skill. They usually grant magical Combat bonuses and are effective against almost every monster - even those that are resistant or immune to physical damage.
An investigator must successfully cast a spell in order to gain its benefits. If an investigator fails to cast a spell it provides no bonuses for the combat at all: this means spells are more powerful and versatile than regular weapons, but are not always a sure thing.
Every spell has a casting modifier, and most have a Sanity cost. To cast a spell, an investigator must pay its Sanity cost and then pass a Spell check. To pay the Sanity cost of a spell, the player simply removes a number of Sanity tokens from his total equal to the cost of the spell. Players must always pay the Sanity cost of a spell, whether or not the investigator subsequently passes the Spell check.
Spells typically last until the end of a combat or the end of a turn; most spells state their duration directly on the card. You can choose to stop using a spell you have cast, but if you do so the spell does not work again for that combat. Once exhausted, spells do not become refreshed until the next Upkeep phase, meaning that most spells only work for a single combat. If you fight multiple monsters in one turn the spell will not be available for another fight.
Spell check: A Spell check is a skill check that uses the investigator’s Lore value, adjusted by the spell’s casting modifier. If the Spell check fails, the spell has no effect. If the check succeeds, the spell takes effect.