Cleansed Crystal Mirror
The Cleansed Crystal Mirror (浄玻璃の鏡 Jouhari no Kagami) is a mirror made of crystal used by the yama to look into people their past.
The mirror is not an ordinary mirror. When you look into the mirror, a yama sees all your past deeds. Everything you have done in the past will be shown and thus there is no privacy. Every little detail may be exposed. With that they can make a fair judgement and either send you to hell or heaven.