In John Gardner's Grendel, the Shaper has a profound impact on Grendel. The biggest reason is that, contrary to the original tale of Beowulf, Grendel has "language." So not only can we comprehend why the creature acts as he does, but we also realize that words can harm the creature.
Grendel is a more sympathetic character in this version of the ancient story. He recounts his lonely existence (except for the presence of the cynical and hateful dragon); the creatures around him are silent. Grendel has been unable to communicate with most of those in his world, leaving him isolated and impressionable.
Enter the Shaper. This blind scop (storyteller) is not only the keeper of Hrothgar's history (the scop's job), but like a propagandist, he is able to change the perception of men—altering the truth so that it is remembered not necessarily as it happened, but as the Shaper recounts it! Grendel notes:
The man had changed the world, had torn up the past by its thick, gnarled roots and had transmuted it, and they who knew the truth remembered it his way—and so did I.
Grendel is made instantly aware of the power of this man. Hrothgar's desire to build a mead hall comes from the words of the Shaper.
...the old man sang of a glorious meadhall whose light would shine to the ends of the ragged world. The thought took seed in Hrothgar's mind. It grew.
Grendel tries to understand this man's strange "power."
He takes what he finds...And by changing men's minds he makes the best of it.
Even more powerfully, the Shaper's words shape the perceptions of the humans about Grendel, who has yet to wage war on mankind:
The harp turned solemn. He told of an ancient feud between two brothers which split all the world between darkness and light. And I, Grendel, was the dark side, he said in effect. The terrible race God cursed.
I believed him. Such was the power of the Shaper's harp!
Grendel is devastated—he enters the hall and begs for friendship with those in Hrothgar's court; frightened, they attack him. He feels more isolated than ever by the Shaper's words. Grendel returns to listen to the old man's songs, and because he was there for the event, he realizes that the man's story of how he raced into the hall is a lie. Grendel admired the scop, but now sees the truth of the scop's words. And so he becomes the monster the Shaper says he is.
The Shaper alters the way Grendel sees himself and how others see him. Grendel realizes that the Shaper weaves words that are, as the dragon says, "illusions." But Grendel is tied to the Shaper somehow. When the Shaper lies dying:
Crouched in the bushes beside the path, peeking in like a whiskered old voyeur, wet-lipped, red-eyed, my chest filled with some meaningless anguish, I watch the old man working up the nerve to let his heart stop.
The Shaper whispers his final words and dies. Grendel's life alters again—for though the Shaper painted him as a monster before he ever acted like one, the poet connected Grendel to the world in the same way he told of valiant heroes and their martial feats. The Shaper's death makes Grendel feel disconnected.
Not only ancient history—the mythical age of the brothers' feud—but my own history one second ago, has vanished utterly, dropped out of existence..."Back there in time" is an allusion of language.
The one who kept their memories alive is gone. In that, Grendel feels he has lost a way to hold onto his own past.
...I strain my memory to regain it.