Brennan was formerly a Satanist, and thus he had a birthmark shaped like three sixes. Brennan also helped deliver the Antichrist when the latter was born of a jackal at a Catholic hospital in Rome on 6 June at 6 AM. At some point, Brennan became riddled with cancer to the point that he had to take morphine twice a day, and began to seek forgiveness from Christ for his former Satanism. Hoping that it would earn him Christ's forgiveness, Brennan became a priest and moved to London to try and warn the Thorns (the Antichrist Damien's adopted parents) that their son was the Antichrist.
Brennan feared evil spirits to the point that he covered his apartment's walls with pages of the Bible, and his apartment's door with crucifixes; in an attempt to keep the evil out. He also followed Damien's adopted father, Robert Thorn, for a long time.
Brennan visits Robert Thorn at the American Embassy one day to warn him about Damien. He tries to get Robert to take communion, and reveals that he saw Damien's birth and knows what his biological mother was. However, Robert then has Father Brennan removed from the Embassy, but not before Brennan reveals to Robert that Damien was born of a jackal.
Ten days later, Brennan meets Robert at an opera and tells him to meet him under the bridge in Bishop's Park at 1:00 the next day; warning him that his wife Katherine is in danger and will die if he doesn't come. The next day, Robert meets Father Brennan under the bridge in Bishop's Park as instructed, and Brennan explains to Robert that Damien is the Son of Satan. He also reveals that Kate is pregnant, but Damien will kill the unborn child, then Kate herself, then finally Robert; and instructs him to go to Tel Megiddo and see Bugenhagen, who can show him how to kill the Antichrist.However, Robert refuses to believe what Brennan has just told him and orders him to stay away from his family, then leaves. Once Robert is gone, Brennan begins glimpsing and hearing things that cause him to retreat through Bishop's Park to a church. He tries to take shelter inside the church, but the door bolts shut; and before Brennan can find another way in, lightning strikes the scaffolding on the church roof. As a result, a steel rod falls from the scaffolding, through a removed stained glass window at Brennan. The rod impales Brennan through the chest while numerous shards of the broken stained glass land in his face and chest; killing him.
- Keith Jennings' photographs of Brennan all showed a rod-shaped anomaly over Brennan, which was more solid and closer to Brennan's body the more recently before Brennan's death they were taken.
Robert strongly hated Brennan for claiming that Damien was the Son of the Devil, and regarded him as insane.