A man with an axe breaks into Castle's PI office before dropping dead. As the investigation continues, Castle becomes increasingly convinced that the Antichrist is behind the killing and that he himself is marked for death.
- The Antichrist: The victim, Gabriel, attacked the Crowne family because he was convinced that their son Victor was the Antichrist, and broke out of his psychiatric facility twenty years later to finish the job. Oscar was convinced too, and nearly succeeded in killing Victor if Castle hadn't intervened.
- Ax-Crazy: The victim appears to be this after breaking into the PI office with a bloody axe.
- Doing In the Wizard: Lanie found sulfur in the victim's stab wound which appeared to be inflicted by a claw. Castle thought a demon was the killer, but the murder weapon turned out to be a gardening claw with traces of sulfurous fertilizer on it.
- Human Notepad: The victim's corpse is covered in tattoos of religious symbols and Bible verses relating to protection from evil. So is Oscar when he's discovered by Castle.
- Jump Scare: Beckett pulls this on Castle a couple of times when he's on edge from supposedly being marked for death.
- Macguffin: In the secret room in Castle's office is the Angel's Dagger, a weapon that can supposedly kill the Antichrist. When Castle goes to retrieve it, he's knocked out by a mysterious assailant (actually Oscar, who really did believe in the occult like his father) who steals the weapon.
- Marked to Die: Castle begins noticing a mysterious mark showing up on his neck every time he shows up on camera. When window washing equipment nearly drops on his head, he's convinced that the Antichrist has marked him for death. Turns out it was just a zipper on his collar catching the light the wrong way.
- Shout-Out: Castle repeatedly notes similarities between this case and the film series The Omen. Victor's life closely resembles that of Damien.
- Schrödinger's Cat: Cited by Castle as the reason why he refused to open the safe until now.
- Secret Room: A mysterious safe embedded in the wall of Castle's office turns out to be the passageway to a secret room containing all sorts of occult artifacts.
