In 37 AD, the ruler of Rome, Gaius Julius Caesar Augustus Germanicus, falls ill. Junius and his family, like the rest of Rome, pray for his recovery. He does......but Gaius himself becomes a depraved monster who, renaming himself "Caligula", slaughters Junius's family in a drunken revelry before going on to plunge Rome into mass violence and anarchy. Junius goes to Rome to seek revenge, but gets dragged into Caligula's inner circle, and as time passes, he becomes at risk of losing his soul.
Aside from the shared title character and historical context, this has no relation to the film of the same name.
- The Ace: Laurentius; he's a talented soldier, an honorable man and relatively accepting of outsiders (his wife is half Jewish).
- And I Must Scream: What happens to the people whose souls the demon steals. The real Caligula also appears to have been subjected to this; at one point he breaks free of the demon's influence and begs Junius to kill him.
- Bad Boss: Caligula barely treats his soldiers better than his victims.
- Bittersweet Ending: On one hand, Caligula is still out there, now possessing Junius, and Laurentius is badly wounded; on the other hand Junius was still able to redeem himself and Caligula's plans were massively set back with the destruction of the heart. His plan to corrupt Laurentius also fails thanks to Junius's intervention, and Laurentius ultimately survives.
- The Caligula: Living up to his namesake, the demon Caligula is a murderously psychopathic dictator who runs Rome into the ground for kicks.
- Card-Carrying Villain: Caligula is perfectly aware of how evil and vile he is, and cheerfully embraces it.
- The Corrupter: Caligula enjoys corrupting innocents into committing atrocities for him, with Junius being his personal pet project, seeking to see how far he can corrupt him before Junius' tries to claim his revenge.
- Corrupt the Cutie: Junius undergoes this, specifically at Caligula's hands.
- Demonic Possession: What happened to Caligula. As he lay dying a demon offered him life; Caligula accepted... and the demon promptly took over.
- FaceHeel Turn: Felix at the end of the first volume. He ultimately does redeem himself in the second volume.
- For the Evulz: Caligula happily commits horrific torture, rape, crucifixions, and just general mass murder, all just because he enjoys being a wicked bastard.
- Historical Hero Upgrade: Agrippina the Younger; while in many ways a conniving bitch she does love Rome and is implied to genuinely care for her uncle (and husband) Claudius.
- Historical Villain Upgrade: Incitatus was probably NOT a demonic talking horse.
- Gaius Julius Caesar Augustus Germanicus AKA Caligula was also probably not possessed by a demon.
- Incest Is Relative: Caligula regularly rapes his own "sisters"; Agrippina the Younger marries (and sleeps with) her uncle Claudius.
- Karma Houdini: The demon Caligula manages to escape both times (though in the second volume his plans suffer a massive setback)
- Kick the Dog: Caligula does this a LOT.
- Kick the Son of a Bitch: Caligula setting Nera up to die would be awful... except Nera herself is a monster who gleefully partook in atrocities with Caligula (including the massacre of Junius's family) and unlike Caligula she didn't have the "excuse" of being possessed.
- Sadist: Getting sexual thrills from violence is as easy as breathing to Caligula, and he is happy to flaunt it.
- Serial Killer: Caligula is a variation of this. While publicly he orders horrific atrocities, in private, he brutally and personally murders and maims innocents before ripping out their souls to "feed" himself, then displays their bodies as "art".