Step into the portal at the base of Konohana to reach the River of the Heavens. The game can be saved at the Origin Mirror before moving north through the area. Follow Issun's instructions to break some pots and wall jump up a ledge before what appears to be a dead-end to the north is reached as the path runs into a body of water. Look to the north-east for a hill with a torii. At the top of the hill, Issun will instruct Amaterasu to draw the missing star in the constellation. To do so, simply dot the left end of the line and she will learn Rejuvenation.
Move back down to the base of the hill and use the new Celestial Brush technique to recreate the stream, scribbling on the space between the two banks. A starry flow will be created, allowing Amaterasu to continue her trek. Obtain the Astral Pouch from the chest before moving through the next portal.
Continue north along the path in the next area until the Cave of Nagi is reached. Stand in front of the large statue and use Rejuvenation to restore Nagi's blade. To do so, draw a line where Nagi's blade should be. There will soon be staring at another incomplete constellation. Fill in the missing dot and Tachigami will emerge, giving Amaterasu Power Slash. Follow Issun's advice to test out the technique on a nearby rock, then use Power Slash to cut down the wooden poles that block the exit of the cave.