The structure is "be + to + verb" and I'd say it's relatively common, especially in formal writing (and sometimes in a literary style) The president is to visit Germany next week. (=the president is going to visit Germany next week (plan)) Your sentence has a different meaning, in that it doesn't suggest a plan. It simply means that their story became one of the best known tragedies, perhaps somewhat unexpectedly to most/some people.
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Hole One a New SeeTheir story was to becomeis to become ~ will become (Future of the Present)