How did beowulf hear about the wrath of grendel?

Individuals had fled to Herot and recounted stories which have traversed the sea more than twelve years.

Grendel is a gigantic animal who stays in the haziness and closed off from society. Grendel ends up plainly enraged by the music and festivity happening in the Hall, hence, making him stir that night and execute the greatest number of as he could.