12 But this aforesaid Antichrist is to come when the times of the Roman empire shall have been fulfilled, and the end of the world is now drawing near. There shall rise up together ten kings of the Romans, reigning in different parts perhaps, but all about the same time; and after these an eleventh, the Antichrist, who by his magical craft shall seize upon the Roman power; and of the kings who reigned before him, three he shall humble, and the remaining seven he shall keep in subjection to himself.
At first indeed he will put on a show of mildness (as though he were a learned and discreet person), and of soberness and benevolence: and by the lying signs and wonders of his magical deceit having beguiled the Jews, as though he were the expected Christ, he shall afterwards be characterized by all kinds of crimes of inhumanity and lawlessness, so as to outdo all unrighteous and ungodly men who have gone before him; displaying against all men, but especially against us Christians, a spirit murderous and most cruel, merciless and crafty. And after perpetrating such things for three years and six months only, he shall be destroyed by the glorious second advent from heaven of the only-begotten Son of God, our Lord and Saviour Jesus, the true Christ, who shall slay Antichrist with the breath of His mouth, and shall deliver him over to the fire of hell.
Source: Catechetical Lectures (New Advent)