Is it possible to interrupt an action with an effect from an enchanted device as if fast-casting? I'm trying to design a device invested with an effect like Repel the Wooden Shafts (ReTe 10) triggered by a command word if the wielder is aware of the attack and beats the attackers initiative total with an initiative roll for the device (as per the rules for fast-casting.) But I can't find any support for doing this in the core rules. Any ideas on how this can be done will be welcome.
I think of an enchanted device as more like a formulaic spell than a spontaneous spell; so in terms of game balance, I think I wouldn't want to be able to "fast-cast" using any old enchanted device. That being said, the wizard who bars incoming missiles by brandishing his staff is a great trope and one that should be realizable. How about making up a new Effect Modification: at a cost of +5 to the effect's level, the effect can be "fast cast"?
As an aside, you might be interested in the Optional Combat Rules in Lords of Men, specifically the Delaying Actions section on pages 118-120. It recasts fast-casting as one example of an Interrupting Action, which is the type of action you're wanting to achieve.
I wonder, and I am not certain of the mechanics of it, but what it you had the Enchanted Device always have RtWS spell active, with the Concentration effect being maintained by the item, and then simply turn it on and off at will?
You can certainly put a constant ReHe ward effect to protect you as with RtWS, but the Base level is 15 instead of 4, before you even take the increased duration into consideration. But I don't know of any way to imitate a fast cast with a magical item. The problem is that you need to direct the effect as you cast it toward the weapon you want to protect against, so you can't just rely on a longer duration to protect you, unless you can see the weapon at least one round in advance.
Possibly you might use a greater enchanted item, with linked effects, one Intelligo to see the weapon coming and trigger the RtWS. Or for that matter, one effect to detect a fast cast spell by you and trigger the RtWS.
Fast Cast is something you can add to items... See Societies, Flambeau. Adds +5 to the effect level.
No. it's "fast trigger" which gives a +3 to initiative. It's not the fastcasting which is a free action by LoM rules, what casting a spell or using an item (slow or quick) is an action.
What about putting a InMe effect in a device with a linked trigger. Activating the device with a thought.
I'm posting here.
The verditius is currently enchanting his talisman, curiously called "Excalibur".
And ten effects (or less but you will soon understand why ten) he wants to add are "Unravelling the fabric of [form]".
The idea is to allow Excalibur to "cut through magic" (well, it's a cosmetic term for dispelling instant magic).
The problematic part is:
1- he could instill a mental power to command the effect, but it's a bit unthematic for the mythic Excalibur sword (well, his is only a copy, but he wants to do something more powerful than the non hermetic original if it ever really existed ^^) to have to read his user thoughts to cut through any magic
2- fast triggering are usable, he could put a lot if needed in the effect, but that wouldn't help with fastcasting.
Actually, for game play reason, the effect is at sight because the user will cancel a magic at that range; but cosmetically, the effect which is coming from the wielder of Excalibur seem to be "cut" by the blade.
To allow Excalibur to be really mystical, it had to be not only on Hermetic magic but also on Hermetic item effects and on unhermetic magical power.
Obviously, it should also unravel "magic" from the four realms.
When we mix all of it, we obtain many interrogation:
- can additional magnitude allow an Perdo Vim general effect to target more than one "category" of effect (spell, effect of item, power of creature are the three obvious ctegory)?
- can additionnal magnitude allow a Vim effect to target more than one realm?
- How would you resolve the thematically appropriate "Excalibur cut magic without needing to read the wielder thoughts to fastcast"?
Of course, the first thing I should have said is that I consider mental communication to trigger an item even faster than fastcasting, since fastcasting require to decide to fastcast then casts a spell but really quickly, although thinking is immediate, and item triggered by linked are triggered by the thought and bypass normal "initiative" consideration, from the time when the user can have thought about activating a power in the timeframe he has (if he is fastcasted by BOAF in his back... he won't be able to thought about fastcasting a PeVi; if it's in front of him, he could).
You can always use the higher guideline for dispelling any effect, rather than the one for dispelling a specific effect. Just because Wind of Mundane Silence says it does not work on Momentary spell does not mean the guideline doesn't, just that the way it is used in Wind of Mundane Silence cannot.
And it has the advantage that the item doesn't have to determine which of its many PeVi effects it should use (or use all of them at once).
Otherwise, yes, I would allow an extra magnitude to add one Form, or realm, or category, or whatever to the list of thing a spell can affect.
Yeah for WoMS i stated that the reason it won't work on momentary is that the effect is constant, and needs at least 6 seconds to adapt itself to a new effect coming in area, meaning a momentary spell is already finished when WoMS would do his effect on it.
The way I'd do it:
First effect: InHe 19. (Constant equivalent of "Shriek of the Impending Shafts" (ArM5, p.136). Serves as trigger for the other two effects.)
Second effect: CrIm 4+ [Base 1, +3 linked trigger]. (Give item a "voice". Serves as range for third effect (as per ArM5, p.112). Add levels for more uses as necessary, with a total effect of 14 being infinite uses.)
Third effect: ReHe 13+. ("Repel the Wooden Shafts" (ArM5, p.138) with linked trigger. Add levels for more uses as above, with a total effect of 23 being infinite uses.)
The item reacts automatically, without "input" from the user. For a "silent" version you'd have to give the item sight as a second effect and make the third effect at sight range.
(Emphasis mine)
I'd personally prefer the sight version, as extending voice range via magic is explicitly not allow, so creating a voice is ... iffy.
It would also have the dual advantages of range and silence.
"Magic items use the wielder’s voice; independent items need to be given a voice (CrIm) to use this range." (ArM5, p.112)
Just for reference. 
good point.
I still prefer the silent version though 
I´ve read the "given a voice" in a rather openended way as needing to make sound rather than specifically a voice.
If a mute magus can make sounds sufficient to reach voice range, hard to see why an item can't do the same.