At which point; immediately after which; used to show that something happened as a result of something else.
Compound of 'where' (from Old English 'hwær') and 'upon' (from Old English 'on'). This formal construction emerged in Middle English to mark sequential relationships in narratives.
Whereupon is practically extinct in modern speech but still thrives in fantasy novels and formal writing—it's the literary marker of 'and then, dramatically.' It's a perfect example of how certain words become 'archived' in specific genres while vanishing from everyday conversation.
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