Slaying Dragons - Second Edition - Preview #2

Exorcists talk about the existence of what they call a "nemesis" to every demon.  This shows why it is important for the exorcist to gain the name of the demon who is possessing someone.

Every demon has a nemesis.  Here's more from the book:
Our Lord also wields His authority over the demons through the use of what Fr. Ripperger referred to as a “nemesis.” When the exorcist gains the name of the demon who is the possessor, he can then invoke the assistance of the demon’s nemesis. The nemesis can be Our Lord or Our Lady under a specific title or one of the saints. Fr. Amorth says that the demons often refer to Our Lady as the “thief of souls” and the saints are called “assassins.” Demons will not pronounce the names of Jesus, Mary, and the saints during an exorcism, unless when under duress and compelled to do so. Fr. Ripperger says that every demon has a nemesis: to Lucifer, it is Christ; to Satan, it is the Father; to Beelzebub, it is the Immaculate Heart. Another example is Saint Catherine of Sienna. She is the nemesis to the “noonday devil,” who admitted this during an exorcism owing to the fact that she conquered him in her earthly life. The noonday devil comes from Psalm 91, and is understood to be the demon of acedia, or spiritual sloth and laziness. 
Fr. Carlos Martins told a fascinating story about an exorcist friend of his, with the pseudonym “Fr. George,” who, during an exorcism, was told by the demon that his nemesis was St. Thomas Beckett. Fr. George then spoke to Fr. Martins the very next day and told him about the demon’s nemesis. Fr. Martins runs “Treasures of the Church,” a renowned organization that travels the world and presents about one hundred and fifty relics of many great saints for the faithful to venerate. Fr. Martins immediately sent a first-class relic of St. Thomas Beckett to the exorcist, who used it in the exorcism. Fr. Martins says that, when the exorcist applied the relic, “the effect was as if all hell broke loose. The demon screamed horribly, as if he was being tortured. In fact, the reaction was so much, that it took Fr. George aback. Under that torture, the demon subsequently revealed that he was the demon who had possessed the king’s men that had murdered Thomas à Beckett.”