The Mist Raven is a Prosthetic Tool in Sekiro: Shadows Die Twice that you can find pretty early in the game. A powerful enemy guards it, but you can just use the same tactic I did, and run away from him. The item is handy, as it allows you to evade enemies that are attacking you by using a powerful Shinobi technique to hide your movements.
How To Find The Mist Raven Prosthetic Tool
In the Hirata Estate, at the Bamboo Slope Thicket Idol, just head up the stairs in front of you. You will quickly come to a tree stump you can grapple to on your left. Jump up there, then down into the river. Swim upstream until you reach a tree stump, past the large bridge. Grapple up, and head through the trees. You will come to a rather pathetic looking bamboo gate blocking a door. Chop it down, then head inside. Head up the sloped corridor, and you will come to a wall that you can jump up.
These walls are interesting because you can jump between them to climb upwards. Do this until you reach the top, then prepare to deal with the Purple Ninja, or be smart like me and just run past him to the building, open the door, and grab the Mist Raven.
Now, the Purple Ninja fight is absolutely something you can do if you wish. I decided that discretion was the better part of valor, and simply ran past him to grab the item. I figured I would simply double back here later and fight him when I was stronger. Frankly, it was still a tough fight when I did come back, and the Purple Ninja once again taught me that there levels to the sword fighting game.
Now that you have the Mist Raven, you can bring it to the Sculptor, and he will add it to your Prosthetic arm, allowing you access to this interesting Shinobi tool.