To overtake with darkness or nightfall; to leave someone in darkness, either literally or figuratively in ignorance.
From be- (prefix meaning 'to cause to be') plus night. This Middle English verb combined Germanic roots to create a poetic term for being caught by darkness, used extensively in older literature.
Benight sounds like it should be a fantasy spell, and in older English literature it was used poetically to describe anyone caught by nightfall or kept in ignorance—the kind of word that sounds modern and magical even though it's centuries old.
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