The Serpent Crystal is a needed item in the game, albeit an unobtainable one. Despite that Amaterasu never uses it, it is beneficial to her quest as it permits her to enter the Moon Cave.
History
Not much is known of the Serpent Crystal, but it seems to have some connection to Orochi as it is able to dispel his barrier. It is briefly mentioned by Waka when Amaterasu meets him again in Taka Pass. Issun, believing Waka to be up to no good, convinces Amaterasu to find it before he does. After much questing and eventually ending up at the Gale Shrine, they explore the shrine until they find it in a round room. However, Crimson Helm shows up and fights Amaterasu, who is aided by the Satomi Power Orbs and Susano.
After the demon is defeated and Susano runs away, Waka suddenly appears, standing rather close to the Serpent Crystal. Issun accuses him of "muscling in on our goods" and has Ammy chase him. Waka leisurely jumps away, telling them that the crystal would be of no use to them. He also informs them that he plans on using the crystal to dispel Orochi's barrier. After giving the pair another prophecy, Waka leaves them in the shrine.
Later on, when Amaterasu is taking Kushi to the Moon Cave, she and Issun find Waka there, meditating intently. He strikes the ground near the barrier with his sword, then pulls out the Serpent Crystal. He places it above his sword and plays his flute, which makes the barrier absorb into the crystal. Waka then tells Amaterasu that she has done her part and to leave the rest to him. However, Orochi interrupts as he takes Kushi into his lair.
Trivia
- The Serpent Crystal has all the elements of Orochi's eight heads.