Answer:
The line 'So Eden sank to grief' is an allusion, or literary reference, to the Biblical story about The Garden of Eden, a perfect paradise until Eve ate from the Tree of Knowledge. By making this reference, Frost is implying that the idea nothing good can last is an old one; it's part of our human experience.
It is John Gardner's novel "Grendel" from 1971 that initiated the trend of telling a story from a monster's point of view. The story is essentially the epic poem Beowulf spoken trough the eyes of Grendel
