Surpassing even The Exorcist, Legion is primarily indebted to the detective novel for its structure, if not for its thematics.

In both novels, Kinderman resembles television's Colombo (also the subject of novels in the 1990s) more than his hardbitten compatriots such as Sam Spade or Philip Marlowe. His character may also have been influenced by Rabbi David Small, in the detective novels of Harry Kemelman published throughout the 1960s and 1970s. Blatty refers to other detectives in Legion, as when Kinderman calls himself "Mr. Moto" and one witness is surnamed Mannix.

The single strongest literary precedent is Red Dragon, by Thomas Harris (1981; see separate entry). Blatty's interviews have not been forthcoming about literary influences. Harris's novels lack the direct philosophical queries of Legion and have even stronger imagery. However, there are two striking similarities. Each is the story of the search for a serial killer, the...