A "fateful moment" refers to an important and significant moment in time that has a major impact on someone's life or the outcome of a situation. Full definition
Many successful people talk about simple, fateful moments when others validated their ambitions. (inc.com)
It was a fateful moment for Shelley Winner, an incarcerated woman at Dublin, a federal prison in California. (cnbc.com)
Depending on which of the many legends you subscribe to, the fateful moment may have happened when a bar of Nestlé semisweet chocolate jittered off a high shelf, fell into an industrial mixer below, and shattered, or when Mrs. Wakefield, in a brilliant move to make her Butter Drop Do cookies a bit sexier, chopped up a bar of chocolate and tossed in the pieces. (nytimes.com)