Hengoku no Schwester

About this title

"Iron Maiden" is an instrument of torture modelled after Mother Mary's form. The interior is full with long nails, and people who are enclosed in it would be pierced to death. But the "Iron Maiden" that exists nowadays is simply a counterfeit made by us based off of old tales. The "maiden" who possesses a gentle, loving face as well as a violent "body"... Did the "Iron Maiden" really exist in the past? This is a story revolving around "her".