Prequel

A prequel is "a literary, dramatic or cinematic work whose story precedes that of a previous work," focusing on events that occur before the original narrative, and it "forms part of a backstory to the preceding work." The word is a twentieth-century coinage joining the prefix pre- (Latin prae, "before") to "sequel"; it first appeared in print in 1922, Anthony Boucher used it in 1958, and it "came into general usage in the 1970s and 1980s." The Magician's Nephew (1955), which shows Narnia's creation, and The Phantom Menace (1999) are the standard examples, and some works are both at once: The Godfather Part II continues one story while preceding another.

The prequel's engine is dramatic irony: prequels "play on the audience's knowledge of what will happen next." That is also its continuity burden, since every fact the original established is canon the prequel must not contradict, which is why a prequel needs the series bible before chapter one. See how to write a series.