This essay focuses on the unrealised compositional work Mysterium by Russian composer Alexander Scriabin (1871-1915) as an example of an imaginary medium. It is demonstrated how the creation of Mysterium was dependent on discursive practices of the time, on the media landscape of 1900 as defined by Kittler, and on a particular development of the local sphere as conceptualised by Peter Sloterdijk. Furthermore, it reveals some ways in which Mysterium interacts with realised and imaginary media of its own time and times to come. The structural complexity of Mysterium is used to discuss the limitations and possibilities of the concept of imaginary media, and the historically loaded Wagnerian concept of Gesamtkunstwerkis employed for historiographical contextualisation. The article thereby provides a critical understanding of the limits of the technological in the concept of media and also offers a new perspective on a historically important but seldom addressed work of art and music, which has often left scholars struggling with its unfinished and unrealised character.