Descending the mostly empty Petřín Funicular, Robert Langdon reflects on the trap at Petřín Tower and worries that the message meant to “isolate” him or Sasha Vesna. Although Sasha proved capable and now has Lt. Pavel’s gun, Langdon feels guilty leaving her alone and hopes Michael Harris has reached her apartment and can provide leads on Katherine Solomon’s disappearance.

Langdon returns to the only concrete clue he has: Katherine’s earlier encoded email, which he had transliterated from Enochian into the seemingly meaningless string “LXXEDOC.” Wondering if the reading direction was reversed, Langdon retransliterates the text backward and gets “CODEXXL.”

Realizing the spacing is wrong, Langdon reframes it as “CODEX XL,” which clicks immediately as a specific reference: the Codex Gigas, the “Devil’s Bible,” currently exhibited in Prague at the Klementinum. He recalls visiting it with Katherine the day before and remembers how the codex’s enormous size inspired a student to nickname it “Codex XL.”

As the funicular nears the bottom, Langdon’s mind runs through what he knows about the artifact: its massive vellum pages, the famous devil illustration, and the legend of a monk who sought Satan’s help to complete an impossible book. He also recalls the codex’s puzzling calligraphy—experts believe it was produced by a single hand with uncanny consistency, defying the time such a manuscript should have taken.

When the train stops, the message’s implication sharpens into a new possibility. With no other clear reason for Katherine to point him there, Langdon wonders if Katherine is at the Klementinum, waiting for him near the Devil’s Bible.