Plate 15 begins with the Bard’s boisterous proclamation of his own divine inspiration. He self-assuredly proclaims, “I am Inspired! I know it is Truth! for I Sing/ According to the inspiration of the Poetic Genius…” (51-2). The Heavens apparently don’t take too kindly to this, as the whole Earth trembles at his arrogance. Milton then rises, himself, and sheds “the robe of the promise, & ungirded himself from the oath of God.”
According to the footnote in the text, Milton rises up “Having recognized himself in the self-deceptive, self-righteous Satan of the Bard’s song.” He sees the failure in his mortal life of bringing his readers closer to God, as “The Nations still/ Follow after the detestable Gods of Priam…” (14-15). So Milton, determined to right his past wrongs, decides, “I will go down to self annihilation and eternal death,/ Lest the Last Judgment come & find me unannihilate/ And I be seiz’d & giv’n into the hands of my own Selfhood” (22-24). In stepping down from his “‘Elect’ status” as the book puts it, he hopes to be prepared (and help others prepare themselves) for the Last Judgment.
Take a look at plate 16, which is not included in the text.


Here, Milton is depicted as a Christ-figure (hence the halo), shedding the aforementioned “robe of the promise”, an allusion to the “garments of salvation” and “robe of righteousness” in Isaiah 61:10 according to the footnote in the text. Remember that Satan in “Milton a Poem” is Milton’s own Selfhood. He must annihilate that selfhood or remain among the Elect in Heaven, cognizant of the fact that he has led so many astray in the cult of personality surrounding him in contemporary England. In his descension, he mirrors the decision Jesus makes in Paradise Lost to sacrifice himself for mankind’s salvation. This is, perhaps, why he is depicted with the Halo.