Answer: The poet, paraphrasing the Bible (2 Kings 19: 35-36). Note that he is NOT describing Sennacherib’s death, but the destruction of his army. Lord Byron knew his Scripture.
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Note also that ‘cohort’ refers to that army, being a (Roman) military unit. It does not mean King Sennacherib’s companion, although the word is now frequently misused as such (i.e. to = companion), probably because of the poem.
Note further that the poem is stunningly beautiful.