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The Holy Mirror was used to prevent the Calamity. It was placed on the "mirror rock" facing the Hell Gate, as the gate was sealed with the blood-soaked ropes from the Strangling Ritual, repelling the Malice.
On December 13th, 1837 the Hell Gate opened and the Holy Mirror was shattered into five pieces that were scattered around the mansion. Miku Hinasaki manages to find all of these pieces and cleanse Kirie Himuro's spirit of the Malice, resealing the Hell Gate.
The last piece of the Holy Mirror was used to create Miku's Camera Obscura.