Who is The Devil from Dark Night of the Soul and what is their importance?

The devil is the ruler over hell and the arch enemy of God. Along with the things of the world and man's own evil nature, the devil is named as one of the three main threats to a believer's union with God. The devil has the ability to use a believer's mind and emotions to lure the believer away from God. For instance, it is noted in the first book that the devil is responsible for placing evil thoughts into the mind of the believer while he is trying to complete spiritual exercises.