Sabimaru is a prosthetic tool that is used to attack the opponents and inflict poison to them. You will find it in the Ashina castle.
Follow the steps below in order to find out the Sabimaru.
How to find Sabimaru:
Go to the upper tower Antechamber in the Ashina castle and then jump down from a wooden bar. There will be a blue samurai, kill him with your sword.
After killing the samurai, move forward into the room and you will find a chest box. Open up and collect the required item.
Now you will be able to craft Sabimaru.