Just like most other endings, stay loyal to Kuro and defeat Owl. Once that’s done, you’ll need both fresh Serpent fruit and an old Serpent fruit. For the latter, head to the idol just before the Guardian Ape arena and turn around. You’ll notice an old lady on the cliff. Speak with her.
She’ll point forward towards the cliff and tell you that the old fruit is forward. You’ll jump forward with a running start and be able to grapple onto a branch below. Keep following this until you find a large cave with a serpent spinning around its crevices.
You’ll run to the back of the cave and notice a small house. You can drop down on a small ledge and get closer to where you need to grapple towards the snake on the leftmost side. Puppeteer ninjutsu on the monkey to distract the snake and grapple up quickly.
From there, enter the house and snag the old fruit on the statue.
If you’re missing the fresh fruit, you’ll want to transport it to the Senpou Temple idol and jump off the cliff directly following this idol. There will be a small enemy and a lever with an attached kite. Puppeteer Ninjutsu by death blowing the enemy twice and he will raise the kite.
Proceed through Monk territory, grappling up into the building and moving past the room with the three monks praying to an undead creature. Once you hop out of the window, there will be a large tree to grapple onto. Afterward, you can grapple onto the kite and fling on a ledge.
On the rightmost side beneath you, there will be several grapple locations. Jump down and grapple quickly. This will transport you to an area where you can deathblow the giant serpent from before. If you want more information, view the Great Serpent achievement.
Once you’ve got the fruit, you’ll want to head back to the Inner Sanctum in Senpou. From there, you’ll want to keep getting rice from the woman. To do this, receive the rice once and then rest outside. Go back in and do it again.
After you’ve exhausted all the dialogue, you’ll be given Rice for Kuro, which you’ll want to deliver to the Heir almost immediately. After doing so, head back to the Inner Sanctum, and you’ll see the woman want a Persimmon now. The apes in Senpou Temple drop them if you’re missing one.
After you’ve done this, you can travel into the Folding Monkeys arena and speak with her again. Head back out and towards the Inner Sanctum once again to speak. She will mention that there is an enemy deep within the caves nearby.
Head to this cave, and you should find a corpse at the end of the tunnel with a scroll. Take the scroll and give it back to this girl. After this, she’ll ask you for the two Serpent fruits we snagged previously. Give them to her and walk back to the idol.
Rest twice. Head back in to speak with her. She will then give you the item for the ending. After you receive it, head through the game like normal, and you should be able to choose the Return ending or the Immortal Severance ending when you beat the last boss.