Grendel is one of three monstrous antagonists, along with Grendel's mother and the dragon, in the Anglo-Saxon epic poem Beowulf (c. 700–1000).
The Old English epic poem Beowulf describes the monster Grendel and his mother in terms that leave little doubt as to their lupine nature - among the words used to describe them are: werga, werhtho, heorowearh, brimwylf, grundwyrgenne, all of which contain the elements wearg/wearh or wylf. This is a complex word: it is often used simply to mean 'wolf', but it also denotes an outlaw or the state of outlawry, in which case it refers to those who have committed crimes that are either unforgivable or unredeemable, and who are cast out from their communities and doomed to wander until they die. Outlaws were traditionally forest-dwellers, and could be legitimately killed. Here, Grendel is referred to as a march-stepper, literally meaning a "boundary-land walker," a walker in outlands or desolate places.
Grendel is also called a scucca (demon), from which the second element of the name of Black Shuck, the supernatural dog encountered by nocturnal travellers in East Anglian folklore, is derived. It is also said of Grendel that him of eagum stod ligge gelicost leoht unfaeger, 'from his eyes shone a fire-like, baleful light'.
Grendel and his mother are both haunters and guardians of a burial mound in marshland, and are given an aquatic aspect to match - brimwylf, for instance, means 'water-wolf'. This brings to mind the bodies of water - usually rivers, but sometimes a lake or sea - that are invariably supposed to surround the Indo-European underworld, and those of some non-Indo-European cultures.
The poem, Beowulf, is contained in the Nowell Codex which tells of the building of Heorot by the Danish King Hrothgar. As noted in lines 106-114 and lines 1260-1267, Grendel's mother and Grendel are descendants of Cain and were regarded as foes of God in the Anglo-Saxon poem Beowulf, which Beowulf leaves Geatland in order to find and destroy Grendel, who has been attacking Heorot. After a long battle, he does so and mortally wounds Grendel. He later engages in a fierce battle with Grendel's mother, who is eventually killed by Beowulf. After 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, "non", about 3pm).  He returns to Heorot, where he is given many gifts by an even more grateful Hroðgar.
Theories / Essays
- In 1936, J.R.R. Tolkien's, Beowulf: the Monsters and the Critics discussed Grendel and the dragon in Beowulf. This essay was the first work of scholarship in which Anglo-Saxon literature was seriously examined for its literary merits—not just scholarship about the origins of the English language as was popular in the 19th century.
- In the following decades, the nature of Grendel's identity would become a conundrum for scholars due in large part to a line where he is described as descended from the biblical Cain, the first murderer. For some scholars, this justifies a monstrous appearance. For others, it positions Grendel as a marginal (rather than monstrous) figure which bears the curse and mark of Cain.
- In 1971, author John Gardner published the novel Grendel, which was the telling of Beowulf from the monster's point of view.
- Kuhn (1979) was the first to raise questions about the association of any of the above images with Grendel and in an essay which would launch fierce (and as of yet unresolved) debates for decades about the term áglaéca:
- There are five disputed instances of áglaéca [three of which are in Beowulf] 649, 1269, 1512...In the first...the referent can be either Beowulf or Grendel. If the poet and his audience felt the word to have two meanings, 'monster,' and 'hero,' the ambiguity would be troublesome; but if by áglaéca they understood a 'fighter,' the ambiguity would be of little consequence, for battle was destined for both Beowulf and Grendel and both were fierce fighters (216-7).
- Other scholars, such as O'Keefe, identify Grendel with 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 suggested that Grendel is a demonized version of the old Danish fertility god Freyr, and even goes as far as linking Grendel with the Green Knight of Arthurian legend.
- Jack, George. Beowulf : A Student Edition. Oxford University Press: New York, 1997.
- Frederick Klaeber, ed. Beowulf and the Fight at Finnsburg. Third ed. Boston: Heath, 1950.
- Kuhn, Sherman M. "Old English Aglaeca-Middle Irish Olach." Linguistic Method : Essays in Honor of Herbert Penzl. Eds. Irmengard Rauch and Gerald F. Carr. The Hague, New York: Mouton Publishers, 1979. 213-30.
- J.R.R. Tolkien, Beowulf, the Monsters and the Critics. (Sir Israel Gollancz Memorial Lecture, British Academy, 1936). First ed. London: Humphrey Milford, 1937.
- Arby Stones, "Hellhounds, Werewolves and the Germanic Underworld"
- Grendel ISBN 0679723110