The St. Benedict Medal is a very powerful sacramental, especially when it comes to protection from devils.
The inscriptions on the front read:
center — Crux s. patris Benedicti (The Cross of our holy father Benedict)
bottom — Ex SM Casino MDCCCLXXX (from holy Monte Casino, 1880)
edge — Eius in obitu nostro praesentia muniamur! (May we be strengthened by his presence in the hour of our death!)
The initials on the back stand for:
cross — Crux sacra sit mihi lux! Nunquam draco sit mihi dux! (May the holy cross be my light! May the dragon never be my guide!)
angles of the cross — Crux s. patris Benedicti (The Cross of our holy father Benedict)
top — Pax (peace)
edge — Vade retro Satana! Nunquam suade mihi vana! Sunt mall quae libas. Ipse venena bibas! (Begone Satan! Never tempt me with your vanities! What you offer me is evil. Drink the poison yourself!)
Sometimes this medal is placed inside a crucifix to combine two powerful sacramentals.
(A valid St. Benedict Medal must be made from metal, not paper, wood, tin foil, or other materials.)
For more info on the meaning and history of the medal, go to: http://www.osb.org/gen/medal.html
See also the book, THE MEDAL OR CROSS OF S. BENEDICT, ITS ORIGIN, MEANING, AND PRIVILEGES BY DOM P. GUÉRANGER, ABBOT OF SOLESMES http://www.liturgialatina.org/benedictine/stbenmedal.htm