Mary's ancestor on her father's side was a bit of an explorer. He loved traveling to far off regions and seeing new things. (Something she has in common with him) And while on an expedition to Hisui he would find this odd keystone and despite the warnings of locals he brought it back to Galar with him...this would be a bad idea...but after some supernatural shenanigans he would have a lifelong friend and house Enett would have a powerful heirloom to pass down to new generations.