I'm confused, why did he get amnesia? And was he a vessel or being protected??

He got amnesia because the birds have been tellin him that the ACTUAL GOD adopted a WHITE HAIRED BABY who is him. He taught he was an ACTUAL GOD where in fact the God resides inside his body. He couldnt accept the fact that the mountain was neglected and him being the Mountain God didnt do anything about it. We can see at the end the ACTUAL GOD leave his body

Thank you!