Beginning a sentence with a relative clause, in other words a subordinate clause, with an antecedent in the previous sentence or paragraph is always strictly speaking incorrect grammar. Writers with a good command of English and style occasionally do so, though. It really makes no difference what the antecedent is in such cases.
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SuumEorumBut from what you said, assuming that the author of the piece had an impeccable command of the language, then it could (perhaps not in the strictest sense of the word) be considered a sentential relative clause?There may be grammarians who might do that. I have never heard the term "sentential relative clause", so I have no opinion about it. A