Familiar Armor

Exploration Stage: Moon Gate
From: Mei
Description: I haven't come across the champion's statuette, but in the heat of battle, I did see a helmet at the Moon Gate. I thought that it must've slipped off Yui's head, but she was still wearing all of her armor. This helmet seems familiar to me... I hope it will help us understand the reason for the monster's appearance. Bring it, please.
Completed: I remember where I saw this helmet! A drawing in the collection of legends. It was worn by one ruler in time immemorial... And he was of our kind!