From Fr. Stephen Rossetti’s “Blog” https://www.catholicexorcism.org/blog
In the midst of an exorcism, quite by chance, a mirror ended up in front of the possessed person. Immediately, the demons reacted violently and screamed to get rid of it. We were surprised by the reaction.
A former practitioner of sorcery and the occult said that his “religion” often hosted underworld parties celebrating particular demons. These demons might take possession of those attending. He said it was common practice to cover the mirrors before the party because “the demon spirits could not look at themselves (or rather the reflection of the human they possessed that night) in the mirrors.”
When possessing a human being, demons experience some of the physical attributes of the energumen. Even though they do not have a physical body, it seems that when they are possessing a human being and looking into a mirror, they see a glimpse of their horrible evil. As I sometimes say to the demons: “God made you beautiful and now look at what you have become!”
Demons are ugly beyond description. No horror movie can capture the repulsive ugliness of sin and the demonic. Demons do not want to hear the truth and they do not want to see the truth. Their reflection in a mirror shows them their ugly truth.
The ritual of an exorcism is about telling the truth to the demons. They have rejected God. They are immersed in evil. Christ has defeated them on the cross. They are damned for all eternity. They will be in an excruciating torment for all time.
I do not know any exorcists who use mirrors. I wonder if it would be a good intervention. Our “weapon” during the spiritual battle of an exorcism is the Truth, who is Jesus. When we pray the Ritual, they hear the truth in words. Perhaps seeing the truth in a mirror might also be good…