Blatty mentions exorcism in Twinkle, Twinkle "Killer" Kane (1967) and its rewrite, The Ninth Configuration (1978; see separate entry), but the closer link to Blatty's other works to Legion is in Kinderman's wit, especially the repartee between Kinderman and Dyer, which resembles much of the dialogue in those novels but is not found in The Exorcist.

(There, Kinderman is just as sharp, but more subdued.) Those books also concern themselves with some of Legion's thematic issues, especially the problem of evil and the hope of some kind of redemption. Self-sacrifice, a theme in The Exorcist and The Ninth Configuration, is replaced in Legion by a unity of all things, so that success is in transcending the self, and individual sacrifice is part of a universal movement of self-salvation.