Spoilers
Remember all the optional boss guides we did in the past for From Software games, because they all have optional bosses? This is kind of like that for Sekiro: Shadows Die Twice.
Location spoilers below, as well as the name of the optional bosses. Everything else is kept vague.
Step one: Progress with the story until you’re nearly at the end
Shortly after fighting the “DD” boss (you’ll know it, it’s after a long mountain climb with lightning strikes in the zen-like area), you’ll enter the endgame of Sekiro.
Several board states change, including the entire area surrounding the main castle (the first zone of the game is now off limits): if you try to progress starting at the main hub you’ll find that the very first branch is burned off. You need to get there from the Ashina Castle, which now has a new bridge that allows you to work backward.
The thing is, most of the castle idols are now offline due to an incident. The easiest way to gain access to the area you need is to come from the Abandoned Dungeon, which is still operational: just head into the outdoor area. Once you’re in the castle you can slowly start to earn back all of the waypoints. Go to the Old Grave and go straight north pointing at the idol, it’s the quickest way.
Alternatively you can enter the bridge connection in a more flashy story-like manner. Go down the castle steps and talk to the wounded soldier at the bottom on the left. They’ll instruct you to find a kite, which swoops you to the bridge after grappling it. If you’re facing the soldier from the perspective of the castle behind you, head left then up on the rooftops and back toward the castle. You’ll find the kite based on smoke trails on the buildings.
Step two: Go back to the beginning, backward
From the Old Grave, locate the newly formed bridge past the small zone.
Head over it and work your way through the enemy mobs all the way into the big gate to the north (ignore the mountain path on the left). Keep going around the curve and eventually you’ll reach a burning village. Forge through that and you’ll hit an idol. That teleports you to the boss arena and another idol you can rest/recover at.
Here you’ll encounter the Demon of Hatred, which nets you a trophy/achievement after clearing it. Note that the idol here is literally right next to the boss, so you can try an unlimited amount of times with instant access.
Bonus round: the other hidden boss
Later in the game one of the Ashina Depths waypoint idols will go offline. Simply return to it with the Mortal Blade item (the fastest way is the Hidden Forest: just turn around and go back into the cave) and beat the boss there. You’ll get the Guardian Ape Immortality Severed trophy/achievement.
You can find out how to unlock the serpent and carp trophies in their corresponding articles.
