The poem Beowulf is contained in the Nowell Codex. As noted in lines 106–114 and lines 1260–1267 of Beowulf, Grendel's mother and Grendel are described as descendants of the Biblical Cain. Beowulf leaves Geatland in order to find and destroy Grendel, who has been attacking Heorot, killing and cannibalising anyone he finds there. Barring his lineage, all motives for his attacks are left up to the reader. Usually in most film or literature adaptions, Grendel attacks the hall having been disturbed by the noise the drunken revellers have made. One cryptic scene, in which Grendel sits in the abandoned hall unable to approach the throne, hints that his motives may be greed or revenge. After a long battle, Beowulf mortally wounds Grendel by ripping his arm off. Grendel dies in his cave under the swamp. Beowulf later engages in a fierce battle with Grendel's mother, over whom he triumphs. Following her death, Beowulf finds Grendel's corpse and removes the head, keeping it as a trophy. Beowulf then returns to the surface and to his men at the "ninth hour" (l. 1600, "nōn", about 3pm). He returns to Heorot, where he is given many gifts by an even more grateful Hroðgar.
Some scholars have linked Grendel's descent from Cain to the monsters and giants of The Cain Tradition.
Seamus Heaney, in his translation of Beowulf, writes in lines 1351–1355 that Grendel is vaguely human in shape, though much larger:
Heaney's translation of lines 1637–1639 also notes that his disembodied head is so large that it takes four men to transport it. Furthermore, in lines 983–89, when Grendel's torn arm is inspected, Heaney describes it as being covered in impenetrable scales and horny growths:
Peter Dickinson (1979) argued that seeing as the considered distinction between man and beast at the time the poem was written was simply man's bipedalism, the given description of Grendel being man-like does not necessarily imply that Grendel is meant to be humanoid, going as far as stating that Grendel could easily have been a bipedal dragon.
Other scholars such as Kuhn (1979) have questioned a monstrous description, stating:
O'Keefe has suggested that Grendel resembles a Berserker, because of numerous associations that seem to point to this possibility.
John Grigsby, in his Beowulf and Grendel :The Truth behind England's oldest legend' suggests that Grendel is a demonized version of the old Norse fertility god Freyr, and even goes as far as linking Grendel with the Green Knight of Arthurian legend.
Grendel has been adapted in a number of different mediums (film, literature, and graphic/illustrated novels or comic books) including the film Beowulf and Grendel, in which Grendel is described as a troll, and is very humanoid, merely being slightly taller and hairier (he is depicted as a child with a beard at the start of the film) than the average human.
In the Robert Zemeckis film, Beowulf, Grendel is more monstrous, resembling a heavily deformed giant covered in sores and scabs. Grendel's attacks are explained in this version by having an oversized eardrum on the outside of his head. The noise of the revelers causes him pain.
He was also voiced by Peter Ustinov in the 1983 Australian animated film, Grendel Grendel Grendel, based on the John Gardner novel. In 2006, Elliot Goldenthal and Julie Taymor premiered an opera of Grendel, also based on Gardner's novel.
Grendel is the name of an ancient, sentient weapon in the video-game Skies of Arcadia. Here, Grendel has the form of a green giant.