Key Item: Shinobi Prosthetic
Item Description: The artificial arm of a shinobi, passed down by the Sculptor. A replica of a human arm fitted with a variety of mechanisms, apparently designed with modification in mind. While it bears a number of cuts and is stained thick with blood and oil, it has been impeccably maintained.
Location: You will receive this item once you lose your fight against Genichiro Ashina in Ashina Reservoir after the cutscene.
Usage: This item becomes your arm and an upgradable weapon. You can find Prosthetic Tools to unlock different weapons at the Sculptor of the Dilapidated Temple.
For all Sekiro: Shadows Die Twice items check out the Sekiro Shadows Die Twice Wiki and Strategy Guide