Phantom Kunai is unlocked later in the game (the earliest you can get it is after reaching Ashina Castle). To unlock this prosthetic tool, you need to own a few other prosthetic tools and their upgrades. You need:
- Loaded Spear
- Loaded Shuriken and Gouging Top upgrade
- Loaded Axe
- Mist Raven and Aged Feather Mist Raven upgrade.
- Anayama the Peddler must be alive (you can't kill him in Hirata Estate; the man wants to loot one of the buildings but he is marked as a neutral NPC). This merchant sells Phantom Kunai (costs 3,000 sen) which is necessary to craft this prosthetic tool.
Phantom Kunai doesn't have any upgrades. It shoots golden butterflies from the prosthetic arm (this costs you 2 Spirit Emblems). This tool works similarly to Loaded Shuriken but there are some differences:
- Butterflies follow the target. However, this attack can be canceled when the butterflies come across an obstacle,
- Butterflies deal a lot of damage to spirits,
- This attack is also useful against guarding enemies,
- Hold the attack button to shoot out more butterflies.