This chapter examines the connections between Joseph Sheridan Le Fanu’s novella “Carmilla” (1871–72) and nightmare-related imagination. The first part focuses on the connections between “Carmilla” and the writing genre of the “case” (be it clinical, judicial, or supernatural). The second part moves to an apparently minor detail featuring in the novella’s conclusion: Baron Vordenburg’s collection of “all the great and little works upon the subject ‘of’ the marvellously authenticated tradition of Vampirism,” from whence Le Fanu mentions four apparently randomly chosen titles. The examination of these texts, and what could be known to Le Fanu, will enable the chapter to discuss in more precise terms the idea of “vampire” as constructed by “Carmilla.” The conclusion undertakes a comparative, “diffractive” reading of “Carmilla” and Carl Theodor Dreyer’s Vampyr (1932), a film famously and explicitly indebted to Le Fanu’s oeuvre. In Donna J. Haraway’s words, “diffraction” can be seen as a modality of generating “more subtle visions” of phenomena by focusing on the loci “where the effects of difference,” rather than similarities, “appear”: in this case, for example, by intentionally inverting the relationship of dependence from “Carmilla” to Vampyr, viewing the film as a device shedding indirect yet revealing light on Le Fanu’s text.