Hi!
I love the whole ArM mechanics and I dislike very much the WFRPG mechanics. On the other hand, I love the Warhammer Fantasy setting.
So, my question for those of you who know the WF setting is, how would you implement the schools of magic of WF using the ArM magic system? Which forms and arts would accessible for each school? and witches and hedge wizards? and Dark Magic? 
I would discard the concept of arts entirely.
Instead I would try to mimic the rules for magic in WHF.
Maybe make each school into a "difficult art" and say that you get to roll, either a die plus your score in that art, and the DC is based on how hard the spell is to cast. Then I would give a a supernatural ability called something like "magic sight" or whatever to represent the ability to see the winds of magic.
If you have the time, could you give me an example with for example the Celestial school?
kinda.
So WHF has 8 winds of magic.
This gives us 8 arts. (forget what I said about difficult arts):
Aqshy - The Red Wind, the Lore of Fire
Azyr - The Blue Wind, the Lore of Heavens
Chamon - The Yellow Wind, the Lore of Metal
Ghur - The Brown Wind, the Lore of Beasts
Ghyran - The Green Wind, the Lore of Life
Hysh - The White Wind, the Lore of Light
Shyish - The Purple Wind, the Lore of Death
Ulgu - The Grey Wind, the Lore of Shadow
for the blue wind/celestial school we get the following descriptions:
power over storms, flight, and mighty hurricanes
and a focus on: Inspiration, Imagination, Exploration.
So spells within the school should have these effects.
Additionally we are told that the celestial school is associated with prophecy and divination.
I cant be botherede to go through the effort of translating spells from WHF into this system. You will just have to take a look at them and give them some approximate levels based on gut feeling and a theoretical examination of the approximate levels in the relevant art, that people will have when they should get the spells. Keep in mind that spell levels should generally be lower when you dont have 2 arts to add to your casting total.
I think I would treat the prophecy part as a mystery virtue and create some initiation rite into the celestial order or whatever, and give the "Visions" virtue. Maybe adjust the rules slightly to make it fit the WHF system more.
the "Magic sight" ability is supposed to be called "Witchsight" I see.
In the warhammer universe magic is supposed to be really dangerous. Fortunately Ars already has a neat mechanic for that: The stress die. However I would rule that casting magic in WHF is always stressful, and that a spell has a base of 3 botch dice +1 for each magnitude of the spell or something. I would also consider tinkering with the rules for reduction of botch dice to make sure that you can never get really low amounts of botch dice .
I would do something cool with warping, maybe keep it as it is, and make sure that twilight scars and other results are in keeping with the school that a caster specializes in.
I would ask @ironboundtome as his blog (which used to Ars Magica focused) is very much WFRP oriented these days, and he has a good overview of both.
Makes my day to see somebody ref the blog - thank you Darkwing!
@Yepesnopes - I've started trying to convert the magic system style into whfp twice and each time it become obvious that the approaches I took would end up as huge tasks.
First was looking to backward convert the existing spells effects into collections of sub-elements and then scale those sub-elements to try to get consistency. eg. what is the wfrp spell Bolt or Levitate actually doing, how would this be broken into micro-components or steps. Then is there commonality across how Aqsh handles the manipulation & manifestation of fire, vs how Hysh handles the same for Light? I was trying to get a feel for how much power and such a CN1 spell has vs a CN6, or CN12 - and then look at each wind as a Talent which is purchased. Language Magic is still used for casting. It got mucky when looking at trying to find commonality across the winds for their relative power and scale.
Second approach was to create a few ballaprk baselines themselves and decide that the new system would not care much for a rule for converting CN of wfrp4e into a SpellLevel to be used against a skill CT.
...Aside - I found that my fellow players and GM in my current group had absolutely no interest in the conversions - which killed them quickly. Many GMs and players in wfrp consider magic deliberately broken and refuse to treat it as anything more than the crude systems of basic dnd. In fact the GM private group for WFRP I watch closely tends to consider magic to be something that is likely to be avoided, and not used by PCs. I see that as weak and limited thinking.
There is a valid counter-point to my obsession with magic in wfrp (and Ars) which is it makes non-wizards feel like secondary characters. Now in ArsMagica that's a feature and not a bug, but in wfrp at mid to high level character XP (say 13,000 xp or higher in my experience) the Wizard can do things that the non-Wizards can only sook about. I'm told it's even worse at 20,000+ XP. My Bright Wizard was just over 13k xp when I stopped playing and it would be considered a fair but not not exceptional Flambeau by comparison in my playing experience using Ars Magica. Lethal in a duel, dangerous to army, but still challenged by a supernatural beast.
My third attempt which I'm still thinking about doing far in the future - will go large - will first look at the meta-physical types of chaotic influence in the world as expressed as spells and unexplained powers, so that the system will work for designing the powers used by the Imperial Colleges, and also for the witches, ogre magi, tomb kings, lusttrian lizards, vampires and such. It might as well incorporate the powers of priests too - because that would be nice as a gather all - but its not really needed, because I separate the divine from the chaos in my headcannon; much like is Ars.
I think lifting the complex skill mesh of spells and spont casting is absolutely needed and desirable, as is abilities like Finesse, Penetration, and in some way I could see fixing the Dawi limited magic resistance in this new system too. I would also like to keep the Verb+Noun style of spell construction, and I think there is good reason to design a cross scale where Elves get access to almost all the Verbs, where Humans only get the basic ones of Create, Destroy, and in this way reflect what Elvish high magic is like, and why its far better than the junk which is taught to human Imperial Battle Wizards.
Writing this post is giving me more ideas again too. Damn it!
So basically I don't think its possible without a lot of work and would need to be built from the group up - and your non-wizard players are likely going to hate it. I should say too that I'm keen to see "the system" be usable in wfrp's mechanics - still use d100 and incorporate opposed tests. Certamen should be added, ramp up corruption, and all sorts.
I had some thoughts of how to "quick fix" WFRP magic without throwing out the current core spell mechanics of wfrp, but they too were seen as making magic too powerful and too flexible. /shrug
Thx for your detailed answer. Actually, there are so many things I dislike from the wfrp 4th ed system that I wouldn’t now where to start, it is not only the magic system (although it is probably the worst).
I’ll keep checking for a system to port the wfrp setting, but ArM is still my preferred system, not only for the magic.
Going the other way (running Wfrp using Ars mechanics) is similar problems in spells - you need to establish what the baseline spell effects are and work up each spell. Chaos botches and Twilight seem similar so perhaps thats an angle to shortcut the work ahead. Still use +Verb +Noun+stat + roll. Perhaps a way to build in the risk and chaos of the setting into magic is to have guidelines force that in the setting? Answers why some chaos spells are able to be powerful but always have penalties and risks.
Techs: Invoke, Perceive, Twist, Destroy, Dominate (same but different)
Form: as said above by Euphemism.
5 = Create a fire inflicting +5 dam (Invoke Aqshy) Just like CrIg
5 = Gain a +2 temporary bonus on next roll (Perceive Heavens)
-5 = reduces level of spell, but caster must check for Twilight/Chaos effect each cast. Stackable. (Witch spell only)
I think that this is a key element, i don't know how i would implement it, but magic in Warhammer is very dangwrous as one is tampering with birderline demonic forces at all times.
I don't think that ars magica 's magic system is a good port for the warhammer universe's magic system, and since the combat system in ArM is probably its weakest point, I am kind of unsure the system would work at all...
It is true. My frame of reference is playing dark heresy, where there is approximately 10% chance of something "interesting" happening when using magic. Though like with rolling a 0 on a stress die in ArM, the 10% of times where a check is made to see if something "interesting" happens, it is far from always that the result is truly catastrophic or even dangerous. Sometimes it causes milk to go sour, sometimes a greater demon is summoned.
What you say is true, but I would offer the following counterpoint: If you want to play a traditional combat-focused WHF game, you are better off hacking a different system to work for you. But if you dont care for combat then ArM has a fine system. Maybe you want to play political intrigue or magic or whatever.
I find very interesting the comments regarding combat. I actually like much more the combat system of ArM than Wfrp and it is one of the reasons I want to change to ArM.
It is deadlier and the way weapons and armours are implemented in the system is much more accurate in ArM than i. WFRP.
The main reasons I don’t like wfrp system and I want to change to ArM is that
in wfrp characteristics have far too much weight when comparec to skills (in ArM is the opposite)
Combat is not as lethal as they sell it (deadlier than DnD of course), in ArM combat is very lethal.
Weapons and particularly Armours are poorly represented (that is my own thing, I like a bit more simulationism approach in this particular point). In ArM they are more historically accurate, although a armour penetration value would be needed.
Since I like those things from ArM, I was thinking if I can use ArM system on wfrp setting, but to do this I would need to port the magic system.
So you see, I did not wanted to port wfrp into Arm for its magic system, but for other reasons 
I mostly remember that system for its flip-a-coin do-you-die trivial checks. It didn't stay on our gaming table very long.
Warhammer AND Ars Magica, two of my favourite things!
So I have pondered this, along with other ways to redesign the Ars magic system for other settings while keeping its strengths.
As @Ironboundtome (love your blog!) says, some of the approaches can be high-effort.
So for my own setting (Scions of Nathas - theres loads on here about it). I decided to import the magic system wholesale but have my characters magic systems differentiated by extra virtues/flaws.
You could run ArsHammer like this.
So a Bright Wizard might have:
While a Celestial Wizard Might have
And so on. You then add a flat debuff for spells that do not involve the focus of the school they study. So a Bright wizard might till be able to make an audible illusion, but its difficult for him. However, making a big burning head relay a message, not so much.
This is the easy route and it requires the GM to keep an eye on how its used because there are all kinds of abuses that sneaky players could use here. But essentially its a low effort fix, suitable for campaigns where your players aren't Ars experts and could struggle with a much larger home rule set.
Option 2 is where it gets spicy.
Here you bin the old forms and instead create 8 new forms based on the 8 different winds of magic.
This will take time, but what you'd need to do is go through the Ars magic guidelines and redistribute them to the new arts. So obviously Azyr would have the weather components of Auram, but also the intellego imaginem effects.
Shyish would have the spooky stuff from life and mentem (ghosts), vim (Spirits) and so on, probably with a focus on the spirits to just the spooky ones.
This would obviously take ages but would be the surest way to get some really good lore=rules stuff.
You'd have to create entirely different systems of magic for the Dwarves (almost 100% crafting). You'd need to change the focus of Elven high magic to allow access to all arts, but that's easy enough.
Addendum to the above.
All the 8 colleges should absolutely be run as mystery cults with further powers available through initiation.
The nature of the colleges seems almost perfect for this.
Thank you for sharing this approach, its actually elegant and solves a lot of problems without inventing everything anew.
btw I'm stealing the name ArsHammer, great work sir!
Love your ideas!!