secret story; untold episode
Refers to a story or episode that is not widely known, often with historical or personal significance. Common in titles like 歴史の秘話 or 皇室の秘話.
I heard the untold story behind the founding of this company.
This book is a collection of secret stories from the Warring States period.
He told a secret story that he had only shared with his family.
He revealed the untold story behind the making of the film.
The long-hidden secret story finally came to light.
裏話 is a casual 'behind-the-scenes' story shared informally, while 秘話 carries a stronger sense of a formerly concealed, often significant, story.
untold story behind the creation; secret origin story
Compound of 秘 (secret) and 話 (story, talk). The reading ひわ is the standard on'yomi combination.