Final answer:
The new subject is the fantastic and diabolical, reflecting the Romantic movement's interest in emotion, imagination, and supernatural themes.
Step-by-step explanation:
The new subject explored during this period is the fantastic and diabolical. This subject gained popularity in the late 18th century and was part of the Romanticism movement, which emphasized emotion, individual imagination, and the exploration of the supernatural.
The Castle of Otranto and The Monk are examples of novels that catered to this new interest in dark, imaginative storylines, which also saw reflection in the visual arts through work by artists like Fuseli. These themes continued to evolve, with romantic artists finding beauty in mysterious and fantastical subjects, eventually influencing the modernist movement which sought new, original, and visionary expressions, often in contrast to traditional norms.