-( Story related, cannot be missed )-
Within the River of the Heavens area, after you've cleared the first area and are climbing the staircase up to the second, you'll eventually enter the Cave of Nagi, the fabled slayer of Orochi from 100 years ago. His blade, Tsukuyomi, will be in disrepair, so use Rejuvenation on it to release Tachigami's constellation.