Ending 4: Return
After obtaining the Mortal Blade in Inner Sanctum, Shelter Stone and the Lotus of the Palace, all of the Sculptor's Idols in the Ashina Castle will become inaccessible. Upon reaching the top of the castle, Owl will be waiting on the rooftop, presenting Wolf the choice to obey the Iron Code, forsaking Kuro or to break the Iron Code and stand by Kuro. Choosing the latter is required for this ending.
Obtaining Holy Chapter: Infested is required for this ending.
If going to Senpou Temple before defeating Genichiro atop the Ashina Castle, the Main Hall Monk will give it to him, asking him to deliver it to the Divine Child of Rejuvenation, otherwise it will be found at the bottom of the lake in Temple Grounds, requiring the Mibu Breathing Technique to reach. Give the book to the Divine Child and take Rice from her (it must be consumed before she can give more) until she becomes sick, then cure her with a Persimmon or Taro Persimmon. She will then give Rice for Kuro, who will, in turn gift Wolf Sweet Rice Balls.
Speak to the Divine Child again, but this time in the Halls of Illusion where she asks Wolf to find the High Monk of Senpou Temple, who is located in a dark cave near the Main Hall. From his body obtain the Holy Chapter: Dragon's Return for the Divine Child.
Wolf must then find the two fruits of the Serpent:
From the Riven Cave Sculptor's Idol, past the Toxic Memorial Mob, head down into a dark cave where the Serpent resides. Follow the path along the right hand side of the cave, taking care to avoid the Serpent's body and the Rock Divers. Upon reaching the head guarding a shrine, drop down onto the left to find a lone Monkey and use the Puppeteer Ninjutsu on it to distract the Serpent and enter the shrine to obtain the Dried Serpent Viscera.
From the first Sculptor's Idol in Senpou Temple, go down to the kite and use the Puppeteer Ninjutsu on the Rat standing next to the mechanism, then head to the large tree before the area with the broken bridge that connects to the Temple Grounds area and grapple to the kite to cross the valley. Continue through the cave and from the overhanging plank, plunge onto the Great Serpent to obtain the Fresh Serpent Viscera.
After obtaining both fruits, return to the Inner Sanctum and give them to the Divine Child to later receive the Frozen Tears.
Once having defeated the final boss Isshin, the Sword Saint, give Kuro the Frozen Tears and the Divine Dragon's Tears and bring him to the Divine Child, who will perform a ritual to become a cradle for Kuro, essentially a vessel. She will then bid farewell to her deceased siblings, departing to the West, to the birthplace of the Divine Dragon with Wolf.