Third person singular present tense of darken; becomes dark or makes something dark.
From darken (dark + -en, causative suffix) + -s (third person singular present). Standard conjugation of the verb darken.
The verb 'darkens' shows up constantly in literature and poetry describing moods and atmospheres—'the sky darkens' is almost a cliché in writing, but it works because it's so fundamental to human experience.
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