This paper introduces the mystical correspondence between two Russian Freemasons regarding a case of communication with an invisible spirit in St. Petersburg in 1819. The correspondence, discovered by the author in the Lanskoi Masonic archive of the Russian State Library, is a unique psychological and cultural document, characteristic of the intense spiritual life of Russian mystics in the 1810s – that is, the ideological and iconographic milieu from which Russian Romantics drew actively. The correspondence opens with a minor Russian Mason’s confession that he has entered into physical contact with an invisible spirit. Almost every day this spirit penetrated his stomach, lived inside him for a while, and caused him lofty and pleasant thoughts and emotions. The body of the man became an object of genuine interest to his more sophisticated instructors who employed a wide range of medical, spiritual, and occult interpretive methods to reveal the nature and plans of the intruder. Was the latter a product of the man’s sick imagination or was it an actual spirit? Was it female? Was it benign or malignant, i.e. was it an elemental spirit or a succuba? Why did it come to him? What attitude should the pious adopt toward this intruder? Why was the story set in present day St. Petersburg? What was its allegorical meaning? In this paper, I consider this correspondence in the context of theosophical (J. Boehme, Mme. Guyon, N.A. Kraevich) and occult literature (Paracelsus, Comte de Gabalis), as well as in the Romantic literary context, in which the theme of the erotic love between a man and an elemental or demonic spirit occupies a central place (J. Cazotte, E.-T.-A. Hoffmann, F. de La Motte Fouqué; V. F. Odoevsky, N.V. Gogol, etc.).