Charms are equipable items that provide passive buffs or unique abilities. You have a limited number of 'Charm Notches' to equip them, and each Charm costs a certain number of notches. Experimenting with different Charm combinations is a core aspect of Hollow Knight's gameplay, allowing you to tailor your playstyle. Some Charms can be 'overcharmed' (equipped even if you lack notches), but this doubles incoming damage.
