The apostle Paul describes being tormented by a "messenger of Satan":
2 Corinthians 12:7: “Because of the surpassing greatness of the revelations, for this reason, to keep me from exalting myself, there was given me a thorn in the flesh, a messenger of Satan to torment me – to keep me from exalting myself!” [NAS]
Some of the theories suggested for the source of Paul's torment include:
- Chronic eye problems,
- Epilepsy, and,
- Someone (such as Alexander the coppersmith, 2 Tim. 4:14).
Of course, there are other reasons cited, but I'm unconvinced that this has been satisfactorily addressed from a spiritual perspective. Of course, it may never be revealed in this life, but I believe it's worthy of further pursuit.