While the Ars Magica world is great, a magic system as good as Ars Magica would benefit a number of worlds.
I am interested in doing an Ars Magica/Cthulhu cross-over, has anyone else done a similar cross-over?
Keith
I suppose it depends on your goals. As I imagine Cthulu, the magic in the world is ritualistic in nature. So if your trying to maintain a Cthulu type world, I'd drop spontaneous and probably Formulaic magic. But that's just me. It all depends on what you want to do.
While I quite like Ars' magic system, I'm not sure it evokes a properly Lovecraftian feel.
Lovecraft's wizards generally end up insane or dead pretty quickly since they spend all their time meddling with Powers Man Was Not Meant To Know; the basic Ars magic system is way too predictable and safe for HPL. (Unknown Armies might work pretty well, though. Guess what you have to do to get a charge!) Now, the only person who would know at this point is Erik, but you might try looking into the Infernal or perhaps the theurgy mystery for magic nasty enough for HPL.
UA is versitle in it's own way. Big fan of tilting. Yeah, I sort of agree that Hermetic Magic just doesn't feel right for Cthulu, but again- that's just me.
I can confirm this-- the diabolists in The Infernal would suit a Lovecraftian story quite well. There's quite a lot of folks dealing with "occult knowledge that man was not meant to know" in there. Also, I imagine that if horror is a strong theme in an Ars Magica story, you're probably dealing with the Infernal realm.
I think it could be interesting as well as bringing the chutulu creatures to the Ars World. Though I think the Ars Magica mages can be a bit to powerful compared to the power of for exmaple mythos mages in CoC, but it can be fun. I have combined Mage and CoC and that worked grate.
A group of courageous Tytalus Investigators vs. the Monster of The Week? 
Maybe 'Ancient Magic' will give us insight into the (Great) Old Ones.
Everyone knows those Bonisagus Seekers are all insane anyways.
I don't think that the mechanics really matter as much as the atmosphere of the game. CoC has a tradition of local magic working. Technically, CoC powers to summon outer gods and blast mortal man isn't magic. It's technology and knowing the secrets of the universe. Semantics sure, but any sufficiently advanced technology... Cthuhlian "magic" could all be based on the character's Ability in Cthulhu Lore. Each summoning spell could require a few experience points, like knowledge of a True Name in RoP: Divine. It doesn't have to use Arts or anything. More than worrying about how spells are cast, you need to address the cosmological impact of introducing Cthulhu to your game.
If you're going to incorporate Cthulhu into ArM, you have to decide which paradigm you're going to use. Are you going to stick with ArM -- Divine is most powerful force in the world -- or are you going to go with the Lovecraftian feel and religion is a cruel joke, mankind is a lab experiment by the Fungi from Yuggoth, and Cthulhu is the most powerful creature on the block. The last one means that, at best, God's just a creature of the Faerie Realm created by the hopes and dreams of humanity. If you stick with the ArM cosmology, then the Great Cthulhu's just the name of some two-bit demon and not much match for anyone using Holy Magic or possessing a few Faith points and a relic or two. This definitely takes away from the horror, and you're basically just borrowing the names of Lovecraft's creatures, which is fine, but not very horrifying in my opinion.
Infernal does a good job of making demons scary, not simple cannon fodder for magi who don't have to worry about the consequences of nuking one. But from a metagame perspective, demons aren't nearly as scary because you always know you can seek the protection of the Big Guy. Divine auras, holy magic, True Faith, etc...are the silver bullet for demons. There's no silver bullet in Lovecraft's world. Knowledge and a trusty Tommy Gun can temporarily stop the plans of the Old Ones, but that's about it.
Personally, I would love to play in a game where most of the Mystery Cults are basically fronts for indoctrination into the Cults of Cthulhu, Azathoth, Nyarlathotep, Hastur, etc. and the rest are dedicated to fighting them. I think springing this on the players as a surprise might really tick some people off though, especially if players think they're signing up for a fantasy game heavy of medieval legends and atmosphere and the storyguide turns the saga into a horror game with sanity checks and futile attempts at staving off the inevitable return of that is not dead which can eternal lie.
You could use the monsters as infernal creatures I guess to keep whit the Ars paradigem, but it dont realy do the mythos creatures justice as many of them are Gods i an of themself.
Well I was going to add a new form "Dimension". To create a portal you need to create a hole in space - Perdo Dimension.
To find a portal, or to determine where it goes - "Intelligo Dimension".
I was going to have a few SAN creation spells. Creo Mentum was my first thoughts, but then I thought about having madness as a desease of the soul rather than the mind, so they became Creo-Vim spells.
I did this because there are so few Creo-Vim spells, and none of them are usefull.
As for wizards dying n ArM, I've lost one.
I would want to keep the Cthulhu Pantheon:Azathoth created the world, and Human kind was created as a joke by the Great Old Ones (if I remember right).
Which means I would need to look again at Realms: Maybe have Evil, Mundane, Magical and Fairie.
And Craimons are insane because Enigmatic Wisdom reduces SAN.
Well I was thinking of having an extension to Hermetic Magic based on a new form "Dimension". (see earlier posts).
Dimensional magic is explained in Books called: "The Teachings of Yog Soggath" and "The Wisdon of Nodens".
Then the Magi can travel though time and space by dimensional portals.
And yes I would warn people, the group I used to play ArM with would have lynched me if I pulled a "Lets go to R'lyeh".
Nope, as good as the ArM paradigm is, I would want to use the Cthulhu paradigm, and the ArM magic system.
I would even set the wizard's home world in Victorian London.
Interesting idea. Some thoughts:
Human Lovecraftian magic was a bunch of set pieces. Formulaic or Ritual; actually, the old Mercurian magic might be the best model, if you have rules for that. More flexible magic might be a matter for Mi-Go or other entities more at home in the Mythos than humans.
Page 215 of the core rules mentions the Old Ones, powerful magical creatures who died/slept millennia ago. Humanoid super-faeries or alien Lovecraftian things? Up to you!
For the theology there are lots of combinations, but yes, the core ArMa and core Lovecraft ideas are incompatible, something has to give. No God, or God is something at the level of Azathoth and Yog-Sothoth, or God is Azathoth (hey, he had a celestial chorus) or Yog-Sothoth ("past, present, future, all are one in Yog-Sothoth') or Nyarlathotep, or God is the supreme "god of Earth" basically a super-faerie, vastly inferior to the Outer Gods. (In Sandman terms, gods of the Dreaming vs. the Endless). Or in the opposite direction, Lovecraft monsters as demons, though I'd find that boring.
I'm ignoring Derleth's additions to the canon, which had friendly Elder Gods opposing the Outer Gods, in a more standard Manichaen manner.
Though note that in Lovecraft -- or at least in CoC's rationalization of it -- there's a distinction between the Outer Gods (cosmic force level) and the Great Old Ones like Cthulhu, more like powerful faeries/demons/magical beings. Possibly even once human-level (not human-like) entities who had transcended through magic and/or technology. I'm sure people have played elder magi with much the same attitude toward grogs as Great Old Ones had.
No role for faeries? Lovecraft's Dreaming is Arcadia. The "little gods of Earth" are the pagan gods.
Cosmic/Eldar seems like Infernal/Divine under different names. And as I said, that dichotomy is Derleth's invention, not Lovecraft proper.
I don't know what to do with auras. Lovecraft did have places of power, so auras and regios seem like they'd fit in. But arguably no friendly auras to humans: power gets boosted, but so does botch dice and Warping, in most or all auras.
Then again, one could argue that the various protagonist spellcasters from Miskatonic were like non-Hermetics with a few abilities. The Gifted would be half-Deep Ones from Innsmouth, or necromancers as in The Case of Charles Dexter Ward.