Sekiro: Where To Find the Loaded Spear Prosthetic
The Loaded Spear prosthetic found in Sekiro: Shadows Die Twice grants players a little more distance, and the ability to remove armor from larger enemies. To locate this prosthetic, you have to go a little out of your way and make sure you acquire a particular key of a guard. Once you have it this item, it'll make fighting a specific boss later on significantly easier.
Loaded Spear Prosthetic
Finding the Gatehouse Key
When locating the gatehouse key, the quickest way of discovering it is to teleport the Sculptor's Idol for the Ashina Castle location. If you haven't already found this, you'll locate it shortly after defeating Gyoubu and after you fight the Bull with the burning hay bale strapped to its head. Proceed inside of the castle, and it should be right at the steps to the palace, right next to the old woman.
Look at the direction the old woman is facing, jump into the pit, using the grapple as soon as possible to get over to the other side. Head on through the open gate, and you should find two guards whom you can eavesdrop on their conversation. After you listen in on their discussion take them out; however you see fit. One of them is going to drop the gatehouse key, granting you access to the building where you're going to the Loaded Spear. But you're not there yet.
Heading to the Gatehouse
While on the bridge where you took out those two guards, you can see where you're going to next. You can get there by returning the way you came, but not going through the gate. Instead, turn to the left, and you'll see a small pathway leading to the side of the bridge. Jump down, grapple to the tree, and proceed forward. Before continuing towards the building with the Loaded Spear, go a bit further and grab the Sculptor's Idol to the left of the pathway, to unlock this area for future use. The Idol should be for the Ashina Reservoir.
After you unlock the Ashina Reservoir, go back the way you came and look for a small opening in the gate, leading off a cliff. If you jump off the side, you should have the opportunity to grapple a tree, and then grapple another, which leaves you facing a Samurai with a gun, facing the other direction. As tempting as it is to jump over and take him out, you want to turn and run up the side of the wall to your left. Get to the top of the roof, and you should see a drummer on the other side. Perform an aerial sneak attack to eliminate him before he alerts the entire area to your presence.
With the drummer out of the way, you're going to see a small building where he was facing. Head on over to the closed entrance, but make sure you're crouched as not to alert other Samurai to your presence. When you approach the doorway, you'll use the Gatehouse key to get inside, where you'll find a chest with the Loaded Spear inside. When you're ready, head back to the Sculptor who will adapt it to your Prosthetic arm, making a future boss that much easier to take down.