I'm thinking about using and expanding upon the information about Beowulf and the Dragon in the 4th edition book Living Legends, and include the covenant Hoerot in Denmark. In Guardians of the Forest, it says that this covenant is plagued by Grendel kin. Now about these creatures: what are they? Are they Magical or Infernal creatures? Or even half-demons?
Any thoughts?
E.
You should see the film Beowulf & Grendel [2005]. In that, Grendel is called a troll and is depicted as a beastly humanoid. This is of course a debatable point and open to interpretation. Some versions of the ancient saga (poem) have called it a dragon.
It is entirely up to you as SG I would say. Any sort of socially repugnant beast could likely serve since the actualities of any legendary terror often change with embellishments to the oral tradition over time.
I have that film, and even though I like it, I don't think that version fits my campaign. I think that in Ars Magica, Grendel should be a clearly supernatural creature. It's only a question if he and his offspring (and his mother) are Magical or Infernal.
E.
Ah, in that case I would likely say magical. Since his chief detractors are mundanes (and a mundane though heroic warrior by legend) magical would carry sufficient cause for claims of diabolic origins.
Personally I found the Beowulf & Grendel film to be quite poignant in its underlying message of the tendency of society to call an outcast evil when in fact its actions toward that society were only justified revenge for original evils done to it and its own by those presuming themselves heroic and good.
At any rate, yes I would say magical. That would leave plenty of room to play with amoral concepts of who is actually good or bad in the bigger picture without the headache of infernal involvements. Your choice entirely though.
Yes, I'm leaning towards Magical too. I'm thinking up a story that links Beowulf's Barrow, the Dragon and Hoerot. It's going to play on the more base human instincts, the lust for glory, greed, passion and violence. 
Sadly, I fear the players in my campaign aren't nearly good enough to be in character to get the most out of this... 
E:
Magical.
Troll-like beasts. or thr lovecraft-like water people, if you prefer "water trolls"
At least this is how I would depict them.
making them neanderthal humanoids would be cool as well, but not very christian, I guess.
Cheers,
Xavi
Did some research, and this is from wikipedia.
"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. Barring his lineage, all motives for his attacks are left up to the reader. 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."
Descendants from Biblical Cain? But in Ancient Magic, Cain himself has Faerie Might?
E.
He could be fairy instead and still contain that notion of the magical since that, after all, is what mundane minds have dreamed forth regarding the magical realm (lacking the understanding to make the appropriate distinction).
That said, this opens a whole encyclopedia of things i would love to wax on about my perspective of the realms and what they actually are but it wouldnt adhere entirely to canon so I shan't atm. Perhaps in some future issue of Sub Rosa I shall set it all down to writing. 
Strange that cain should be considered Fairy, though, since he is in Biblical terms, merely a mundane man, however cursed he may be. Certainly the Church would argue his natural origins even if some legendary suppositions might have linked his curse to vampirism or other possible fates.
If you decide to stick with magical, then you are in essence confirming his tangible and fixed nature as a mythic beast (if you follow my line of thought), misunderstood (amoral) or truly malevolent as you desire him. If you go with Fairy, then you open up the possibility of a mutable being based on what society has imagined him to be at any given time. This is equally playable and fun, so again it's up to you.
I would say Faerie. Grendel (and his fictional children) has a function in the story, which is to be slain by the hero/es. Additionally (and this is purely personal) I have very few human-shaped magical creatures.
However, demonic creatures work as well - how about a group of Furies?
Mark
Woudln't the Furies, being as they are mythologically linked to the gods, be considered Fairy beings in the canon?
Sorry, I was talking about the Order of Furies (the Furiae) from RoP:tI.
I would agree that the Erinnyes (=Greek Furies) would be something entirely different, but I would vote for Magic. They are an uncaring personification of vengeance against those who trangress social boundaries. This puts them in the Magical Spirit camp, IMO. The key point is that they were never worshipped, and are impersonal. They don't punish wrongs done to themselves. They also hark from an earlier generation of 'gods'; which as HoH: S mentions, might suggest that they predate the usurption by the Faerie Gods (this latter is conjecture, however, on the part of House Tytalus).
Mark
Social, human.... that puts the erynes in the FAERIE camp straight on, as far as I can see. Difficult to get more human-integgrated than social interactions here....
Xavi
First off, don't rely on a movie to give you the flavour of the story. Movies are great for visual imagery, but invariably suffer a sea-change thru the eyes of the writer, director, actors, etc etc. Seamus Heany (Irish poet laureate) recently translated a version that is quite "user-friendly" for an epic poem, one that uses alliteration (several similar sounds in sequences) rather than rhyming - much truer to the olde Aenglish sound, and recommended.
But as Trolls, and an ancient line of creatures, I'd say Faerie, distinctly on the dark side. In the final tally, fae or magik is a judgement call, as I doubt there was such a distinction in Anglo-Saxon culture (and thus cannot be definitively translated into Ars terms).
Making the dragon magical and the trolls fae gives your magi a broader spectrum of problems to deal with, too.