When meaning is unclear, it is important to analyze a quotation in context with the rest of the text [I've inserted the whole contextual quote above]. You must ask questions like: Who is speaking? What is the subject spoken of? Is there a conflict, a resolution, a conclusion, a need, a paradox stated?
In this quotation, the speaker is Lucifer, or Satan, also called by Milton "the lost Archangel." He and his legions have just fallen from Heaven to the newly made Hell. Lucifer is adjusting his psychological reactions to their new state of being.
"Is this the region, this the soil, the clime,"
Said then the lost Archangel, "this the seat
That we must change for Heaven?—this mournful gloom
For that celestial light? Be it so,..."
He is saying farewell to the joys and happy environs of Heaven and hello to the misery and "horrors" that now surround him. Milton comes close to giving Lucifer a note of remorse or regret at what his rebellion has cost him, though the hint of remorse is immediately...
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