For sheer virtuosity, it doesn't compares with Moore's "The Loves of the Angels," which is on a similar theme, though it treats the subject from a different angle.
Byron's depiction of Japhet's doomed love for a daughter of Cain is moving and sensitive. Some of the choruses of doomed mortals at the end are pretty good, too. I wish he'd written the rest of it, but, alas, I will have to paint the end myself in imagination.
Byron makes a few references without showing off his source material. The names of the two fallen seraphs, Samiasa and Azaziel (Shemihaza and Azazel) are the names of the leaders of the Watchers in the Book of Enoch. Azazel's name appears in the Bible, in Leviticus, and in the King James is translated "scapegoat," though it appears as a personal name in newer translations such as the NRSV and, I believe, the NAB as well. This is apparently the name of some demon living in the desert, associated with goats. He shows up in noncanonical Jewish texts, most famously in Enoch as one of the angels who chased after women. Azazel also plays a role in DC Comics and has probably appeared in other fantasies, though I can't think of any offhand.
One of Byron's characters, Oholibamah, makes reference to Cain being conceived in Eden, though Genesis would have his conception shortly after the Fall. In all likelihood, Byron is alluding to the mythic tradition that has Cain as offspring of Eve and the Serpent rather than Eve and Adam. Milton uses the same notion in Paradise Lost
, though more subtly. Byron also rejects the gloss, which Milton accepts, that the "sons of God" in Genesis 6 were offspring of the line of Seth while the "daughters of men" with whom they had children were offspring of the line of Cain; Byron has Oholibamah saying specifically that the Sethites and Cainites have never intermarried. Moore, on the other hand, is careful to defend this gloss in his endnotes even as he's defying it in his poem.Madeline L'Engle's novel Many Waters
may reference "Heaven and Earth"; at any rate, she chose some of the same names, though her characters and situations are markedly different, though there is some interrelationship in the characters' roles. Perhaps her novel is meant to be in dialogue with Byron's work. Notably, though she has a Namah and an Oholibamah, Azazel and Shemihaza are absent, though a fallen angel named Eblis, whose name I think comes from the Koran, is present. L'Engle, for whatever reason, seems to scrupulously ignore Enoch for her novel even though it's the most obvious source material for this subject.A few poems in English on this theme came out around the same time, and all are difficult to access. I've found Byron's and Moore's, but can anybody tell me how to get a copy of Ebenezer Elliott's Spirits and Men or James Montgomery's "World Before the Flood"? I'd be much obliged.
Update: Digitized copy of World Before the Flood discovered! I know what I'm doing tomorrow....
