St. Michael the Archangel,
defend us in battle.
Be our defense against the wickedness and snares of the Devil.
May God rebuke him, we humbly pray,
and do thou,
O Prince of the heavenly hosts,
by the power of God,
thrust into hell Satan,
and all the evil spirits,
who prowl about the world
seeking the ruin of souls. Amen..

Origin of the St Michael Prayer

According to legend, Pope Leo XIII (reigned from 1878-1903) composed the now famous "Prayer to Saint Michael" after celebrating the Holy Sacrifice of the Mass with a group of cardinals. During the Mass, the Holy Father fell to the floor at the foot of the altar. It seemed that the Holy Father had died or suffered from a stroke. Suddenly, the Pope revived and said, "What a horrible vision I was allowed to see."

Apparently the pontiff saw a future influx of demonic forces into the Catholic Church. He subsequently authored the following prayer to Saint Michael, seeking to gain further protection for the Church. Pope Leo XIII also ordered this prayer it be prayed by the priest and faithful at the end of every low Mass.

O glorious prince St. Michael,
 chief and commander of the heavenly hosts,
 guardian of souls, vanquisher of rebel spirits, 
servant in the house of the Divine King 
and our admirable conductor,
 you who shine with excellence 
and superhuman virtue deliver us from all evil
, who turn to you with confidence 
and enable us by your gracious protection
 to serve God more and more faithfully every day.