In Beowulf, how does Beowulf find out about Grendel?
The answer to this question can be found after the first narration of Grendel's attacking the mead hall of Hrothgar and the chaos he thus causes, which of course oppresses Hrothgar and his people and makes them fear that they will never be able to be free of Grendel's tyranny and grip of fear over their lives. We are told, however, that news of this situation is brought to King Hygelac of the Geats by Danish sailors:
This heard in his home Hygelac's thane,
great among Geats, of Grendel's doings.
It is Hygelac's nephew, Beowulf, who chooses to respond to the implicit challenge of such news by going to Hrothgar's aid with some loyal men and challenging Grendel. Beowulf has already made himself famous for monster-killing and for his impressive feats of strength, and so obviously relishes the chance to test his skill and gain fame by slaying Grendel.