Today is the feast of St. Michael the archangel who cast Satan out of heaven. (Rev. 12:7-9)
Most Catholics are familiar with the short St. Michael prayer. Here is the original longer version which was written by Pope Leo XIII after he received a vision of the demonic spirits attacking the church. (more info here)
O GLORIOUS Prince of the heavenly host, Saint Michael the Archangel, defend us in the battle and fearful warfare that we are waging against the principalities and powers, against the rulers of this world of darkness, against the evil spirits. Come thou to the assistance of men, whom Almighty God created immortal, making them in His own image and likeness and redeeming them at a great price from the tyranny of Satan. Fight this day the battle of the Lord with thy legions of holy Angels, even as of old thou didst fight against Lucifer, the leader of the proud spirits and all his rebel angels, who were powerless to stand against thee, neither was their place found any more in heaven. And that great dragon was cast forth, the ancient serpent, who is called the devil and satan, who seduces the whole world; and he was cast forth upon Earth, and his angels with him.
But behold! the ancient enemy of mankind and a murderer from the beginning has been fiercely aroused. Changing himself into an angel of light, he goes about with the whole multitude of the wicked spirits to invade the earth and blot out the Name of God and of His Christ, to plunder, to slay, and to consign to eternal damnation the souls that have been destined for a crown of everlasting life. This wicked serpent, like an unclean torrent, pours into men of depraved minds and corrupt hearts the poison of his malice, the spirit of lying, impiety and blasphemy, and the deadly breath of impurity and every form of vice and iniquity.
Be favorable to Thy Church, the Bride of the Lamb without spot, whose enemies have filled to overflowing with gall and inebriated with wormwood. They have laid profane hands upon Her most sacred treasures. Where the See of the most blessed Peter and the Chair of Truth has been constituted as a light to the nations, there they have placed a throne of their abomination and impiety; so that with the Pastor struck, they may prevail to disperse the flock.
Therefore, most invincible Leader, be with the people of God against this spiritual wickedness and bring about victory. Thou art venerated by Holy Church as Her guard and patron; Thou art glorified as our defender against the impious powers of earth and of hell. Unto thee the Lord hath handed over the souls of the redeemed to be placed in happiness above. Entreat the God of peace, to obliterate satan beneath our feet, lest he prevail further to hold men captive, and to injure the Church. Offer our prayers in the sight of the Most High, so that the mercy of the Lord may swiftly overtake us, and apprehend the dragon, the ancient serpent, who is the devil and satan, and send him bound into the abyss, so that he may seduce the nations no more. Henceforth having been confided to thy escort and protection, by the authority of the Holy Mother Church [if recited by a cleric of the order of exorcist or above, say instead: by our authority], do undertake to repel the infestations of diabolical deceit in the Name of Jesus Christ, Our God and Lord.
V. Behold the Cross of the Lord (make the sign of the cross); flee bands of enemies.
R. The Lion of the tribe of Juda, root of David, has conquered. V. Let Thy mercy be upon us, O Lord.
R. As we have hoped in Thee.
V. O Lord, hear my prayer.
R. And let my cry come unto Thee.
[If recited by at least a deacon add the following:] V. The Lord be with you.
R. And with thy spirit.
Let us Pray.
O God, and Father of Our Lord Jesus Christ, we invoke Thy Holy Name, and we humbly implore Thy clemency so that, through the intercession of Mary, the Immaculate ever-Virgin Mother of God, of blessed Michael the Archangel, of blessed Joseph, the Spouse of the same blessed Virgin, of the blessed Apostles Peter and Paul, and of all the Saints, Thou may deign to offer us aid against satan, and all the other unclean spirits, who wander through the world to injure the human race and to destroy souls. We ask this through Christ Our Lord. Amen.