Denethor appears in a fictional novel that began as a sequel to what novel?
Denethor: Denethor II, son of Ecthelion II, is a fictional character in J. R. R. Tolkien's "The Return of the King", which is the third and final part of his novel "The Lord of the Rings". The Lord of the Rings:  The story began as a sequel to Tolkien's 1937 fantasy novel "The Hobbit", but eventually developed into a much larger work.
The Hobbit