While exploring Dunwall you will also find Bone Charms, a lesser version of Runes that will enable you to boost some of your abilities. You can only equip a limited number of Bone Charms at any given time, though this limit can be increased during the course of the game.
Bone Charms provide small supernatural benefits. Locate them by listening for the "song" they emit. By default, you can activate up to 3 Bone Charms at once in the Bone Charms section of the journal.
