First mentioned in Hammered by Leif when Atticus asks him if there are any vampires that are older than either of them. At the time, Leif says he has never met Theophilus and only knows rumors he has gathered. Among these are that he can walk in the daylight when it's overcast, and that he divides his time between Vancouver, the Australian town Gordonvale, and the Greek city Thessaloniki, saying he "follows the clouds." It is Leif's theory that Theophilus is from Thessaloniki originally. The irony of a vampire with a name that means "loved by God" is not lost on Atticus, though Leif mentions it may just be a name he chose for himself and not his original name.
Atticus's first interaction with Theophilus occurs during Trapped, when Leif suddenly appears in Greece, explaining that he was "forced" into his service. He gives Atticus a cellphone which then rings. Theophilus speaks on the phone to Atticus in Latin, and when asked about the ancient Roman pogrom against the druids, admits to being responsible. He claims the the goal was to expand the vampires' territory using the Romans as a means to an end, and that while there were many involved in the plot, it was Theophilus's idea. The reason for the pogrom was the ability of druids to unbind vampires on sight with great ease, no matter how powerful the vampire was. He then offers Atticus a deal, that he will let him live if he agrees to not hunt vampires or train any more druids. Atticus sarcastically accepts, testing his reaction, but is soon after attacked by dark elves apparently in Theophilus's employee, leading him to conclude that the call was merely intended as a distraction.
Leif later informs Atticus that Theophilus has immense technological resources, including the ability to intercept and trace phone calls, which was how Leif located him. He also tells him about Werner Drasche, Theophilius's supposed right-hand man.
In Staked, Atticus directly pursues Theophilus. During a phone conversation with Leif, he boldly asking for his location. Leif, having been cast out of the Theophilus's inner circle, speculates that he is a still at a specific hotel in Prague. Unbeknownst to both of them, Drasche is eavesdropping on their call and sets up an ambush, which Atticus barely escapes alive. Later, Atticus enlists the help of Flidais to track down Theophilus's real location, which turns out to be the Monbijou Hotel in Berlin, Germany. Atticus makes a daring raid using a magical stake given to him buy Lucta that unbinds vampires if it so much as breaks their skin, instead of needing to be driven into the heart. He manages to kill 19 of the vampires there through a combination of the stake and his own spoken magic, but Theophilus escapes largely by chance. Leif speculates that he likely is headed to Rome, his primary residence where the original pogrom against the Druids was launched via the Roman Empire.
A final confrontation occurs at the Piazza di Spagna, with Atticus, Granuaile and Owen all armed with Lucta's magical stakes, and assisted by members of the Hammer of God. A fierce battle ensues, with Theophilus eventually appearing in person for the first time. He sucker-punches Atticus, breaking his jaw and preventing him from speaking his unbinding spell. Just as Theophilus is about to kill Atticus, Granuaile throws her stake at him and pierces his side, unbinding him into a puddle of gore.