"Oh, you need a Gatestone already?" Yuki's Mother asks with a proud smile - "I always knew that you had an exceptional talent!"
"Well, I have your father Kado's Gatestone and was going to pass it down to you one day, but to be honest, I'm not quite done using it myself yet."
"But your father told me where he found this Gatestone when he was a young man; and believe it or not, this Gatestone has a twin."
Here, Yuki's mother pauses, looking Yuki and Miya up and down, as if evaluating their competence or something.
Satisfied with what she sees apparently, she then continues - "Along the rocky cliffs of the East Ocean, north of Sendai, is a large sea cave. Inside that cave is a shrine to the Ocean God."
"Your father told me that he was once shipwrecked off that coast, and washed up into the sea cave, where he discovered the shrine. Unfortunately, a great centipede demon - 'Oni-Makude' feeds off the spirit energy that spills from the shrine."
"Your father said that it attacked him, as he prayed at the shrine to offer thanks to the Ocean for sparing him from the shipwreck. In the ensuing fight, your father knocked out one of the demon's eyes and escaped with it. The eye of the demon is his Gatestone."
"So..." Yuki's mother finishes, "If you need your own Gatestone, I suppose you could always go find that shrine and take the demon's other eye. Though it would probably just be best to kill the demon outright this time I suppose..."