Sekiro: Shadows Die Twice - Every Ending Explained (And How To Get Them)
4. Purification - How To Get It
Here's where things start getting complicated. For the most part, the Purification ending can be triggered by following the same path as the regular Immortal Severance option. However, after defeating the Great Owl you need to return to Kuro's quarters, slip around the side of the room to the screen doors and eavesdrop on the Lord. Then nip out to Isshin's room and eavesdrop on him and Emma.
Rest, and then head upstairs in Kuro's room to talk to Emma, who'll tell you that Kuro can't die and there needs to be another way to sever his immortal ties. Agree, rest, and talk to Emma again, who says she can't find anything. Then, travel to the Old Grave and talk to her there (she's next to the headstones to the left of the idol). Afterwards, head to the Dilapidated Temple, go around the back and eavesdrop on Emma and the sculptor. Finally, confront her and she'll give you Father's Bell Charm.
This will open up a whole new version of the Hirata Estate memory, where you'll have to fight through a bunch of tough mini-bosses and eventually take on the Great Owl once again. The fight is one of the best (and most difficult) in the game, but killing him will net you the Everblossom Branch.
Afterwards, kill the Divine Dragon (make sure you leave this boss to last, as killing it before unlocking the new memory will lock you out of the previous steps), kill Genichiro and the Sword Saint, and pick the option to give the Dragon Tears and the Everblossom to the dying Kuro.