The Book's End.Why would she be revived with that kind of spell?Right before she died, she cast the spell on herself.Wha...?By rewinding time, the object's "position and circumstances" are reset to the point when the spell was cast.The research diaries were coded, too.The Book's End is a time spell.The Book's End...for a while I wasn't able to grasp the true nature of the spell.But after hearing that Sakuya was the (planned) first volunteer in the research, everything clicked into place.