origin story

[ awr-i-jin stawr-ee, or-i-jin ]
/ ˈɔr ɪ dʒɪn ˌstɔr i, ˈɒr ɪ dʒɪn /

noun

a backstory, or established background narrative, that informs the identity and motivations of heroes and villains in a comic book or similar fictional work: The superhero’s origin story begins with a tragic accident that left him scarred, but also resulted in his supernatural powers.
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019