Silk is Hornet's source of energy in Hollow Knight: Silksong, and is similar to SOUL which the Knight uses in Hollow Knight.
In-game Silk is represented on the HUD by a bar with several notches. Hornet starts with 8 notches of Silk and can acquire more throughout her journey, up to a maximum of 15.
Usage/Abilities
Silk can be used to Bind, quickly healing one Mask of damage for each set of 4 notches of Silk.
Grapple: Hornet charges up briefly before using 2 notches of Silk to throw her Needle forward. A string of Silk connected the Needle to Hornet and if the Needle hits a a piece of the environment, she travels to that location. This ability can also be used on an enemy, damaging them as well. Hitting an enemy with this ability restores Silk. The ability can be used multiple times in a row, without landing on the ground, so long as there is Silk available.
Hornet's Battle stance
Gossamer Storm: While airborne, Hornet uses 5 notches of Silk to cause threads of Silk to lash out around her in an area of effect. This is similar to the effect of the Thorns of Agony Charm in Hollow Knight. This attack does not restore Silk.
Silk Spear: Hornet uses 5 notches of Silk to attach a thread to her Needle, throwing it out and pulling it back in quick succession. This attack can hit multiple enemies and does not restore Silk.
Battle Stance: Hornet can also perform a taunt, similar to the start of her boss battles in Hollow Knight. She bends down, with her head forward, and a Silk thread rapidly moves around her. Though this visually uses Silk, it does not deplete her Silk meter.
Acquiring Silk
Attacking Enemies
Enemies struck with the Needle cause Hornet to gain Silk at a rate of 2 notches per hit.
Silk Spindles
Silk Spindles are found across all of Pharloom. When hit with the Needle, they release strands of Silk that are absorbed, granting 1 Notch of Silk each time they are struck. After a certain number of hits, the Silk Spindle runs out and no longer gives Silk. They are similar to Hollow Knight's Soul Totems.