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.
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.
the speaker
Bramble of Destruction.
Sheen spears stars sea
Synecdoche: a substitution of a part of something for the whole or the whole for a part.
Sennacherib was a wanton murderer who sanctioned the leveling and razing of cities, the torture, flagellation, mutilation, and desecration of both living and dead, and the deportation of hundreds of thousands if not millions of subjects of conquered kingdoms. If he were alive today, he would be on trial for numerous genocides and other crimes.
That depends when you're asking about. One answer is the downfall of Sennacherib at the outskirts of Jerusalem, about 135 years before the First Destruction.
In the Bible, specifically in 2 Kings 18-19 and Isaiah 36-37, Sennacherib is the Assyrian king who besieged Jerusalem, but there is no direct mention of rats in these accounts. However, historical records from Assyrian texts do reference the use of rats as symbols of disease and pestilence during sieges. While rats may not be explicitly connected to Sennacherib's story in the Bible, they are often associated with the broader themes of destruction and hardship during ancient warfare.
That was Sennacherib.
in assyria
It occurred when God sent His angel of death to kill 185,000 Assyrian soldiers who were encamped around Jerusalem during the reign of King Hezekiah. You can read about it in Isaiah 37, verses 36 to 38.
Sennacherib
Explaining the speaker's experiences as a military commander.