in th' assembly next upstood
Nisroc, of Principalities the prime;
As one he stood escap't from cruel fight,
Sore toild, his riv'n Armes to havoc hewn,
And cloudie in aspect thus answering spake. [ 450 ]
Deliverer from new Lords, leader to free
Enjoyment of our right as Gods; yet hard
For Gods, and too unequal work we find
Against unequal arms to fight in paine,
Against unpaind, impassive; from which evil [ 455 ]
Ruin must needs ensue; for what availes
Valour or strength, though matchless, quelld with pain
Which all subdues, and makes remiss the hands
Of Mightiest. Sense of pleasure we may well
Spare out of life perhaps, and not repine, [ 460 ]
But live content, which is the calmest life:
But pain is perfet miserie, the worst
Of evils, and excessive, overturnes
Clearly he's averse to pain, although perfectly happy to join the revolt. A quick search turns up the information that he was a god of agriculture, that his name probably signified "eagle," and that he was associated via the Hebrew word "neser," which referred to a plank of wood that King Sennacherib was told came from Noah's Ark. In good pagan form Sennacherib proceeded to worship the plank of wood as an idol.
In fact he was worshipping that very piece of wood when he was murdered by his two sons.
36So Sennacherib king of Assyria departed, and went and returned, and dwelt at Nineveh.
37And it came to pass, as he was worshipping in the house of Nisroch his god, that Adrammelech and Sharezer his sons smote him with the sword: and they escaped into the land of Armenia. And Esarhaddon his son reigned in his stead. 2 Kings 19.36-37
Adramelech earlier were associated with the blood sacrifice of young children, here this god does nothing even as the king worshiping him is being murdered by his own children. The story also fits a deep motif in Milton -- the stupidity of idolatry. First Sennacherib was dumb enough to believe some con artist's tale of a tub about a plank, then he bows to its worship, getting assassinated for his pains. (In some versions of the story, his sons kill him by toppling a lamassu -- a heavy stone idol, also purportedly a "protective" deity -- on him.) Finally there's the thread fundamental to Book 6 of the relation of father and sons, originator and offspring, central in so many ways to the poem.