This is the first tool you’ll find and so far, the most versatile. It’s a ranged weapon that will allow you to instantly “shoot” down softer targets like wolves, lizards, insects, etc. Further upgrades of this weapon will allow you to even damage enemies while they’re blocking your shuriken. By default, shurikens can also deal extra damage to enemies hit in mid-air. Fortunately, you can get this tool early in the game. Each use of this tool costs 1 Spirit Emblem.
From the Ashina Outskirts - Gate Path sculptor’s idol, there’s a gate with a broken wall/window on the upper floor.
Just grapple there and enter. Look to the left to find a body. Pick up the Shuriken Wheel beside him.
Return to the Sculptor back in the Dilapidated Temple and have him fit this tool to your prosthetic. Once done, don’t forget to equip it from the equipment menu to be able to use it.
You’ll access to the prosthetic tool upgrade menu after defeating Gyoubu Oniwa and getting the Mechanical Barrel he drops. Talk to the Sculptor and give him the barrel to unlock the menu.
Similar to skills, there are also upgraded versions of prosthetic tools that will require pre-requisite upgrades before you can unlock them. Each upgrade will require upgrade materials; higher tier upgrades requiring uncommon or rarer materials. Take note that you can still use the older versions of the tools should you wish to.