Past perfect because of the combination of the auxiliary "had" and the past tense of the verb "fall", "fallen".
Hey there!
Your answer is "In this brief fraction of a moment they take the first step toward performing a metamorphosis that changes people from a group into a mob."
Foreshadowing is a literary element that writers use to give a sign about a future event that may occur. Therefore, this is the correct answer, because it tells about a future event or gives a sign about.
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