Do demons exist?

When demons showed up, St. Faustina knew what to do.

Turn to any page of St. Faustina's Diary, and you'll find spiritual gems, like this one:

Today, the Lord came to me and said, My daughter, help Me to save souls. You will go to a dying sinner, and you will continue to recite the chaplet, and in this way you will obtain for him trust in My mercy, for he is already in despair (Diary, 1797).

Suddenly, I found myself in a strange cottage where an elderly man was dying amidst great torments. All about the bed was a multitude of demons and the family, who were crying. When I began to pray, the spirits of darkness fled, with hissing and threats directed at me. The soul became calm and, filled with trust, rested in the Lord.

At the same moment, I found myself again in my own room. How this happens ... I do not know (Diary, 1798).