Epigraphs are like little appetizers to the great entrée of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"And if the Babe is born a Boy / He's given to a Woman old, / Who nails him down upon a rock, / Catches his shrieks in cups of gold."
Gardner chooses a quatrain from late 18th-century visionary poet William Blake's "The Mental Traveller." It's a long, abstract, allegorical poem that personifies the cycle of birth, death and rebirth in the characters of an infant boy and an old hag.
So how does all this connect with Grendel?
Well, we can certainly recognize the interest in the cycle of the seasons that we see in Grendel. But there's something a bit deeper and darker going on here.
Perhaps the image of the shrieking baby that Gardner focuses on in the epigraph represents the sacrificial scapegoat—the person who has to suffer and die to atone for the sins of his society and make it whole again. In that case, Gardner is giving us an early clue as to what he thinks the monster's role really is. Maybe that role isn't to torment the supposedly good humans; maybe, as the dragon suggests, the monster has to be marked off as a bad guy so that humans can think they're superior.