In the courtyard ahead, interacting with the Black Soul allows you to see past phantoms.
Follow the phantom and approach the door ahead. Your gauntlet will glow blue. At this point, press V to use "Eye Awakening," which reveals hidden objects.
Use Eye Awakening, then turn around and strike this small spider. Once the spider is defeated, the door ahead will open.
There are quite a few Phantom Demons in the courtyard ahead. Take them out. Enemies with their backs turned can be assassinated directly.
A new system called "Clash of Blades" appears during combat. When locked in a standoff with an enemy (indicated by a close-up clash animation), you can switch attack methods to deal damage. Simply put, if you trigger the Clash of Blades with a left-click attack, counter with the R key; if you trigger it with the R key, counter with the left mouse button.
Continue forward and eliminate the enemies in the courtyard. Don't go left for now; keep going straight.
