There are different kinds of lines. The imagery in these lines evokes a sense of Mystery.
A sense of mystery is simply referred to as anything that is kept hidden or remains a secret. Example is the mysteries of nature. any affair, etc. This tends to raise curiosity or different kinds off speculation.
The use of the word darkness in both lines shows that something is not seen and the use of the words 'settles' and 'calls' implies that the speaker is present in the darkness.
Darkness settles on roofs and walls, But the sea, the sea in darkness calls; The imagery in these lines evokes a sense of laziness. fear. mystery. despair.
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