Sekiro: Where To Find the Sabimaru Prosthetic Tool
Sekiro: Shadows Die Twice may only have the main character using one primary weapon, but they allowed the Prosthetic Tool to come with a variety of different options, granting the player a slight feeling of diversity. One of these useful tools is the Sabimaru, a short sword used for those who prefer fast, quick-firing attacks. Not only are these attacks ideal for handling the most annoying of foes, but the weapon also deals poison damage, making it great for those who have died to these effect several times over. Here's how you get it.
To find it, you need to have access to the Ashina Castle. When you're there, you need to proceed upon the roofs of the palace and attempt to get on top of the main castle, overlooking the entire area. If you're on the correct path, you should find the Upper Tower - Antechamber idol, right next to a window. Pray here, and you'll be ready to go forward.
Leap of Faith
There's a Samurai in blue garb right ahead of the idol, so when you're proceeding inside, remain cautious and take him quietly if you can. When you're in the next room, you should see two Samurai with blue garb again, and an old woman, who looks like a priest. Take them out if you like. However, what you need to do is jump into the pit they're patrolling. Jump down, and you should find a grappling point in the middle of the room. Grab onto it, and look down.
You'll see three enemies down there. Two are standard Samurai, and the third is one of the more robust, blue clothed foes you've already met. Target whichever one you want, and then leap down, taking out your targetted opponent with a stealth kill, and deal with the two survivors. After you kill them, proceed into the room with the items, and you'll find the Sabimaru inside of a chest, facing the doorway you entered. You'll have to turn around, open the main gates to the Ashina Castle, and go back down to the Ashina Castle Idol to return to the Sculptor.