Nail

Nail is the term used to describe the bladed weapons wielded by several enemies, NPCs, and bosses in Hollow Knight, including the Knight.

The Nailsmith in the City of Tears can upgrade the Knight's Nail up to 4 times if given enough Geo and Pale Ore, which will increase the damage of melee strikes.

The charms Fragile Strength/ Unbreakable Strength and Fury of the Fallen will also increase Nail damage.

Nail-bouncing
By jumping and slashing down with the nail on environment jugs, spikes or enemies, the Knight will bounce. This move is known as "nail-bouncing" or "pogo-jumping," useful for fighting some enemies and for platforming.

In Fungal Wastes, there are pink mushrooms on which the Knight can nail-bounce higher than usual. They must be used as a platform to access the Mantis Village and other locations before obtaining Mantis Claw, Monarch Wings, or Isma's Tear.

Asides from these mushrooms, there are few other times where nail-bouncing is required:
 * To obtain a Vessel Fragment in Deepnest by nail-bouncing on Garpedes.
 * To complete some parts of the White Palace and the Path of Pain.

Trivia

 * Upgrading the nail does not change its appearance in the Knight's sprites.

Гвоздь Aiguillon 骨钉 Ostrze Aguijón Nagel