In reference to #2's post on modern superheros and their foes:
Mentioning Superman makes me immediately think of the infamous Death of Superman arc, in which the semi-mythical hero -- Superman (Beowulf) -- comes up against a force as powerful as himself -- Doomsday (Grendel). In defending his adopted city Metropolis (Heorot) Superman defeats the monster but dies himself. Early in the story, Doomsday escapes from his prison underground but one of his arms remains tied behind his back, mirroring Grendel's missing arm. Superman is aided by a band of other heroes and must eventually wage war against an even mighter force, Mongul (the Dragon). The narrative doesn't match up perfectly but it can easily be read as a modern Beowulf tale.
In fantasy movies there is virtually invariably the wicked evil villain, sometimes in the form of a monster, as Grendel was also. I think of a British TV series based on an age old favorite tale that traces the life of a teen-aged Merlin who comes up against the evil, imprisoned dragon and the dragon's minions of evil. This fantasy certainly, though a TV series and not a movie, has parallels with Beowulf.
How about approaching a different kind of modern film? When I first read your question, my thoughts immediately went to a new genre of horror flick that focuses on gore, the kind that are very appropriately rated NC-17.
The focus on the heroic in these very early English stories and epic poems had to include a fair swath of awesome gore such as the scene when Grendel ravages the soldiers in the hall. Why my mind went directly to The Hills Have Eyes where a mutant mutilates people in a mobile home, I'll never know. (Makes me question my darker side, actually.) Still, I can see the connection.
Humanity is somehow attracted to this kind of violence. Twas always thus, it seems. As Anglo-Saxons of old gathered around to hear the scops relive dramatic poetry about bloody battles, ... modern Americans pay to go to the movies and relive those same bloody battles on screen. Go figure. I love 'em, too.
Beowulf is a famous hero versus monster story. It follows the hero's journey megamyth to a certain extent. It is one of the earliest written English stories, so naturally it has served as a model for others since.
As others have pointed out, the Old English epic Beowulf uses many archetypal motifs, especially that of the young hero in general but also those of the young hero who becomes an old, wise king; the hero who fights and slays monsters but is eventually overcome by death; the hero who, by assisting another people, helps overcome frictions between two lands; the hero whose people prove undependable; and many others. Beowulf is a hero not only because of his enormous physical strength but also because of his wisdom and his virtue.
The basic theme is universal and dates back to the stories told around the campfire in prehistory. Human nature tries to explain events and the forces causing those events. When rational explanations can not be found, supernatural powers are identified and personified as super-human or monstrously powerful creatures because that's how our limited human understanding can try to get a grasp on what is being described.
I agree with #3. There is a sense in which this text picks up on the archetype of good vs. bad, which of course we see in Beowulf and the evil Grendel. This is something that we see echoed in countless books, films and poems throughout the ages. Many fantasy works use this concept of having an ultimate evil that is defied by the forces of ultimate good, and this text is merely another example of this age-old theme.
The motifs introduced in Beowulf follow the plots of many literary narratives. The slaying of monsters in order to preserve territories and fair maidens has become the subject of many a classic work. For, Beowulf is the consummate hero, the concept that lasts to this day from the Arthurian legends to the Lord of the Rings.
By the way, Beowulf has been made into a movie and is soon availale on DVD as a Robert Semeckis film.
I am confused about what "account" you are referring to given you do not mention what Grendel and Beowulf have in common with fantasy and adventure today.
If you are referring to the Anglo-Saxon epic Beowulf itself, the text has influenced modern fantasy and adventure today. Not only has the tale of Beowulf and Grendel been used in at least two feature length movies, Beowulf and Beowulf and Grendel, the idea of the hero and their monstrous foe have been depicted numerous times in fantasy and adventure. One could look at the modern "heroes" of Superman, Spider-Man, and Batman all could have been influenced by characters such as Beowulf. The foes of these characters are often depicted as monstrous and dark (like Grendel).