Satan suggests that he and Beelzebub travel to the “deary plain,” described in line 180, so that they may do what?
Line 180 states: "Seest thou yon dreary Plain, forlorn and wilde, The seat of desolation, voyd of light."
In the opening of Paradise Lost, Satan and his horde have already been thrown out of heaven and landed in hell. They find themselves imprisoned with the Lake of Fire. When they get their wits about them, Satan and Beezebub begin to discuss what to do next. Continuing to wage open war is not, however, an option. In lines 143-5, Beelzebub notes that God, is "our Conquerour, (whom I now / Of force believe Almighty, since no less / Then such could have or'pow'rd such force as ours)." They've openly acknowledged that they cannot beat him, so they start brainstorming other means to retaliate.
At this point, Satan sees the "dreary plain" a distance off, and says:
Thither let us tend...
Save what the glimmering of these livid flames
Casts pale and dreadful? Thither let us tend
From off the tossing of these fiery waves,
There rest, if any rest can harbour there,
And reassembling our afflicted Powers,
Consult how we may henceforth most offend
Our Enemy, our own loss how repair,
How overcome this dire Calamity,
What reinforcement we may gain from Hope,
If not what resolution from despare. (181-91)
He is suggesting they go to the plain and rest if they can, reassemble the survivors of the battle, and discuss what methods are now left to them to "most offend" God. They seek ways to make the best of a bad situation: how can they possibly regroup after this loss (188)? If there is any hope to be had--probably not--how can they draw strength from it? If there is nothing left but despair, how can they use it to drive their anger?