untold story behind the creation; secret origin story
Refers to the hidden or little-known background of how something came to be, often used for products, works of art, or events. Carries a slightly dramatic or promotional tone.
Hearing the untold story behind this movie made me like it even more.
The secret origin story of the new product was revealed on the official website.
Compound of 誕生 (birth, creation) and 秘話 (secret story). The exact origin of the compound is uncertain, but it likely emerged in modern Japanese media to describe the hidden background of notable creations.