The following St. Michael Prayer is the original version as written by Pope Leo XIII. It was composed after he experienced a vision on October 13, 1884. After Mass, the Pope paused at the foot of the altar and lapsed into what looked like a coma. After he recovered, he related the terrifying vision he had seen of the battle between the Church and Satan to occur in the 20th century and composed the following prayer:

Prayer to St. Michael
 
"O Glorious Archangel St. Michael, Prince of the heavenly host, be our defense in the terrible warfare which we carry on against principalities and powers, against the rulers of this world of darkness, spirits of evil. Come to the aid of man, whom God created immortal, made in His own image and likeness, and redeemed at a great price from the tyranny of the devil.
　
Fight this day the battle of the Lord, together with the holy angels, as
already you have fought the leader of the proud angels, Lucifer, and his
apostate host, who were powerless to resist you, nor was there place for
them any longer in heaven. That cruel, that ancient serpent, who is called
the devil or Satan who seduces the whole world, was cast into the abyss with
his angels.
　
Behold, this primeval enemy and slayer of men has taken courage. Transformed
into an angel of light, he wanders about with all the multitude of wicked
spirits, invading the earth in order to blot out the name of God and of His
Christ, to seize upon, slay and cast into eternal perdition souls destined
for the crown of eternal glory. This wicked dragon pours out, as a most
impure flood, the venom of his malice on men of depraved mind and corrupt
heart, the spirit of lying, of impiety, of blasphemy, and the pestilent
breath of impurity, and of every vice and　　iniquity. These most crafty
enemies have filled and inebriated with gall and bitterness the Church, the
spouse of the immaculate Lamb, and have laid impious hands on her most
sacred possessions. In the Holy Place itself, where has been set up the See
of the most holy Peter and the Chair of Truth for the light of the world,
they have raised the throne of their abominable impiety, with the iniquitous
design that when the Pastor has been struck, the sheep may be scattered..

Arise then, O invincible Prince, bring help against the attacks of the lost
spirits to the people of God, and give them the victory. They venerate thee
as their protector and patron; in thee holy Church glories as her defense
against the malicious power of hell; to thee has God entrusted the souls of
men to be established in heavenly beatitude. Oh, pray to the God of peace
that He may put Satan under our feet, so far conquered that he may no longer
be able to hold men in captivity and harm the Church. Offer our prayers in
the sight of the Most High, so that they may quickly conciliate the mercies
of the Lord; and beating down the dragon, the ancient serpent, who is the
devil and Satan, do thou again make him captive in the abyss, that he may no
longer seduce the nations. Amen.

V. Behold the Cross of the Lord; be scattered you hostile powers.
R. The Lion of the tribe of Juda has conquered, the root of David.
　
V. Let Your mercies be upon us, O Lord.
R.: As we have hoped in You.
　
V. O Lord, hear my prayer.　
R. And let my cry come unto You.　
　
Let us pray.
　
O God, the Father of our Lord Jesus Christ, we call upon Your holy name, and
as supplicants we implore Your clemency, that by the intercession of Mary,
ever Virgin immaculate and our Mother, and of the glorious Archangel St.
Michael, You would deign to help us against Satan and all other unclean
spirits, who wander about the world for the injury of the human race and the
ruin of souls. Amen.