Silk

Silk is Silksong's answer to Hollow Knight's SOUL, but the flow is reversed: you fill your spool by landing needle hits on enemies, so offense fuels everything else. You spend Silk to heal with Bind and to cast Silk Skills like Silkspear and Cross Stitch. Your maximum Silk grows by collecting Spool Fragments scattered through Pharloom, every two of which add a notch, and three rare Silk Hearts from the Old Hearts questline give major Silk upgrades. Because the same bar both heals you and powers your skills, managing Silk is the central tension of every fight.