The idea that a supernatural being always has to choose an anagram of her (or his) name in her new "incarnation" dates back to J. Sheridan Le Fanu's classic 1860s novella, Carmilla. Carmilla has had other lives as Millarca and Mircalla, if memory serves. The author of Rosemary's Baby very cleverly recycled this idea in that nove and, of course, in the popular film which was making headlines in 1968 when these shows were written.
This episode includes one of my all time favorite DS lines: "Danielle Roget was perhaps the most EEE-vil woman the Eighteenth Century ever produced!" There's something so deliciously camp and over-the-top in that line. And of course, Thayer delivered it with superb aplomb!
G.