Which Tolkien story is described as the earliest Middle-earth tale in his legendarium?

The Fall of Gondolin is described as the earliest Middle-earth tale Tolkien wrote for his legendarium.

Tolkien began writing the story in 1917, while recovering from illness during the First World War. It recounts the hidden Elven city of Gondolin, its king Turgon, and the city’s eventual betrayal and destruction by the forces of Morgoth. The tale became one of the central legends of Tolkien’s First Age mythology.

The story existed in multiple forms across Tolkien’s lifetime. An early version was included in The Book of Lost Tales, while later drafts changed names, details, and narrative emphasis. Unlike The Hobbit and The Lord of the Rings, it was not completed and published as a full novel during Tolkien’s lifetime.

Christopher Tolkien edited a standalone version, The Fall of Gondolin, published in 2018. That book presents the story through selected versions and commentary rather than pretending that Tolkien left one definitive final manuscript. Its long development shows how Tolkien continually reshaped his mythology.