Amano-Iwato is the legendary cave where the sun goddess, Amaterasu, hid when she was maddened by Susanoo-no-Mikoto. At the entrance of the mountain is a stone tablet with the story of the cave engraved in it:
Susanoo, the Japanese god of the seas, was the one who drove Amaterasu into Ame-no-Iwato. This caused the sun to hide for a long period of time.
In order to get Amaterasu out of the cave the other gods threw a party outside. When she heard the noise she got curious and peeked but got blinded by the mirror Yata no Kagami and so she thought they were celebrating the arrival of an even greater and brighter goddess than herself. Though in reality it was her own mirror image. Then Tajikarao forced the cave to open up the rest and the world was bathed in light once again. As Amaterasu stepped out of the cave a holy seal was applied to it so that she couldn't go back into hiding.
You took a look at the cave and noticed that the way down is blocked by a large boulder with a glowing seal on it. So the legend is true, a holy seal was put it in so that Amaterasu, or even other gods or people may venture in it. As you go back, you see something shining at the ground. You pick it up. It was a magatama necklace. Want to take and wear it? (If so, put it on your character's profile page.)