Mervyn Peake’s 1947 novel Titus Groan continually defies genre recognition. Scholar Farah Mendlesohn, however, places it decisively in the category of immersive fantasy. Building on Mendlesohn’s categorization, I further illustrate Peake’s mastery of immersive fantasy in Titus Groan by analysing a symbol imbedded in the text: a green light. This article reaches across media to frame Peake’s descriptive strategies in the context of his role as a visual artist, particularly in his use of abstracted imagery to depict a collective mental and emotional experience between his characters. This, paired with prophetic language surrounding the green light, gestures to the presence of spiritual forces in Peake’s fictional world. I begin by outlining the art historical context in which Peake worked, concentrating on the philosophies of Peake’s artistic contemporaries and the connections between abstract art and notions of universal spiritual forces. I then closely analyse the green light in Titus Groan, linking Peake’s descriptive mode to these philosophies. Finally, I explore narrative evidence for connecting these abstract descriptions of the green light to a spiritual agent at work in the novel—an assertion which heightens an understanding of Peake’s meticulous worldbuilding. Pulling from fantasy scholarship, art historical writings, and inter-artistic theory, this cross-media study argues that Titus Groan, in addition to its oft-viewed physical setting and vibrant characters, houses an observable spiritual plane—into which the green light provides an entry point for the reader. The first study to identify Titus Groan’s green light as a repeated and significant symbol, this article bolsters an understanding of Peake as an immersive fantasy worldbuilder by focusing on the first book of the Gormenghast trilogy. With its narrow focus on Titus Groan and the symbol of the green light, this article alerts scholars to the existence of Peake’s unobserved worldbuilding exertions and forms a basis for further study of the trilogy.