Mirror of Redirection (Form)
ReVi (General)
Duration: Momentary
Target: Individual
Range: Voice
Targetted
When cast against an oncoming spell of the correct form it will "bounce" the spell at a target specified by the defending magus.
The bounced spell is exact in all details except that it will hit a different target.
Will work on spells upto half of (level-5).
(reposted from an ABE thread, but my idea)
Looks good to me, but I wonder at Penetration. A level 25 version of this will be able to redirect spells of lvl 10 or less. However, and I don't know cunterspelling that well, it seems that defeating the spell would be quite difficult.
Consider:
Jerbiton with no special Art virtues, 0 stamina, has a ReTe casting total of, say, 40 (20+20). Launches a rock at your shield man (Unseen sling -- level 10). He believes you might do something so he does cast with Penetration of say +35 (high Pen skill -- 5). Assuming your magus guesses the right Form in time and FAST CASTS so that he can effect the Jerbiton's spell, I think he has to cast the level 25 version with a +36 penetration to save his grog. Factoring in fast casting, he or she has to be able to put together a ReVi casting total of 71. I don't have my books with me, but I think a Terram requisite would be required as well.
Possible? Sure, for a very senior magus or one with a Major Magical Focus in counterspelling or the like. But a more efficient approach would to be to change the rock into water (since it is a lower level effect and thus more likely to penetrate) and let your grog get a little damp, momentarily.
The ReVi guidelines on p161 do not mention needing to penetrate. Why do you believe that this spell needs a higher penetration score than the attacking spell?
(That's an honest question, not disagreement)
It sounds like he's saying the original magus is putting more power into the spell than just the level of the spell...therefore it should be harder to mess with his spell since it's essentially a higher level.
For figuring the level of the spell you could assume it's like a device...add 1 level for every 2(?) penetration. eg: a level 20 spell cast with 10 penetration would be considered a level 25 spell.
I'm watching this thread with keen interest. I'm inclined, though, to ask the same question Erik posed. If the spell somehow first was resisted and thereafter redirected I'd buy the reasoning right away. But the way it's described, it sounds more like the effect of the spell is "changing" one of the spell's parameters, one target for another. Couldn't the spell's level in itself serve as the barrier you'd have to break in order to affect the spell?
Probably to match the MuVi guidelines for casting on hostile magic, and I would agree (on that at least). The ReVi guidelines and spells do not have any examples for spells used on hostile magic (nor any direct alterations to any element of a spell--they change things around a spell but not the spell itself).
Although I like the spell, I'm not sure this spell is totally Kosher--I see the author looking to be in line with the guidelines for MuVi (which is cool to use when there are no guidelines). However, due to the changes the spell makes to another spell, and due to the differences between the ReVi guidelines vs the MuVi guidelines, I really think this spell is Muto not Rego (except like many things in MuVi, it's prohibited in a real stingy way). Unfortunately for the author, the MuVi guidelines specifically prohibit changing the target even for the best effect. ReVi guidelines don't have any suggestions there either, nor any of the metamagical effects suggested with MuVi. The spell is targetting another spell and causing it to act in an unnatural way. Then there's also a question about altering magic that has a duration attached that the targetting spell should have a similar duration.
I suspect that within the rules, I'd have to be a real jerk and suggest a MuVi (with a PeVi requisite to affect a spell of sight or greater range), and instead of altering the target, I'd say force a change of range. I'd keep the level the same. This spell might actually require a minor breakthrough, sucky as that is. The guidelines aren't very nice in this area. Is that fun? No... so I might let things slide a bit... However, by the rules in the strictest sense, I'd have to deny the spell as written.
Further, should the spell be specific to a form? Quite possibly, but who knows--there are no guidelines there (except for PeVi, and you could use those I suppose, giving a bonus for specificity).
Also, the spell doesn't follow the MuVi guidelines exactly ("up to" instead of "less than"), but the guidelines probably have a typo so no worries on that.
Sorry to be a spoil sport.
-Mario
I disagree that this spell isn't "Kosher."
I think this is more simplistic version of "Mirror of Opposition (Form)," as it changes merely the target of a spell rather than its Form.
While not specifically written in black and white in the "Guidelines" section, they are not the only guidelines for spell invention.
While I can agree that this is a Muto Vim spell, I wouldn't require a breakthrough or anything for it. After all, if changing the very form of a spell is a feat you can perform with MuVi spells, then changing a single parameter like target shouldn't really be an issue.
Ultimately, it is the call of the storyguide to allow this spell (as some spells, while "legal" must be stricken from use for game balance). Personally, I would, and make the Level of the "Mirror of Redirection (Form)" the required "Penetration" for the magus casting the hostile spell.
EDIT: Actually, I'm rethinking the whole issue of Penetration. I'll get back to this thread after some thought about it.
Hold on, I have it. I did say "may not" 
. In fact, it is a MuVi, and easier and clearer than we thought. There are, however, a few bits we need to attend to (if playing by the rules in the strictest way).
I read the gudelines for "Total Change" as those are the ones that seem to be used in the description. That guideline specifically does not include changes of target. However, at a closer look, the guideline for "Significant Change" does allow for a change of Target.
The MuVi guidelines explcitly state that the penetration of the cast spell must exceed the penetration of the targetted spell, and that the spell must be fast cast.
MuVi is highly restricted in certain ways. As to if this is fair or cool... YMMV. Their guidelines are horrendously restrictive compared to a bit of vis and a 1pt virtue. However, as written, the spell would have to exceed the penetration of the targetted spell.
There is a minor discrepency between the guidelines in the book and most of the spells in the book--the guidelines say "less than" and the spells usually say "up to" (or equivalent). The new spell designed retains this discrepency.
No idea if it needs to be form specific or not, as the MuVi rules are indiscriminate here--some are and some are not.
The spell would then be
Mirror of Redirection (Form?)
MuVi (General)
Duration: Momentary
Target: Individual
Range: Voice
Targetted
When cast against an oncoming spell (of the correct form?) it will "bounce" the spell at a target specified by the defending magus.
The bounced spell is exact in all details except that it will hit a different target allowed for in the original spell. Will work on spells up to one magnitide less than this spell.
Question: as the change is a muto spell, could the change always be instantaneous, or in the case of a spell with a longer duration, would the change have to have a matching duration (as described in the errata and clarifications for MuVi spells)?
Very Nice.
As for the last part, my final ruling would be:
As the spell merely changes the target of the spell, all parameters of the original spell remain unchanged, including the original magus' casting total and penetration or other factors.
It's like a package, unopened with all its original contents and just had a different address label slapped on it and forwarded to a new address. The CONTENT remains unchanged. It was just sent somewhere else.
You may be making an instantaneous change to a spell, but now that the effect has reached it's target, "Target" is no longer an issue.
OK, I'm seeing this work now. Still a high price to pay, but if your magus is Vim specialist with decent Muto and Rego scores, his opponents better be very slick and very fast if they sling any spells in his direction. A knife in the back is probably what he worries most about.
I think if you match or exceed the duration of the targeted spell, that should be sufficient. Mostly for no other reason than that it lets you get on with the game session rather than arguing about whether it takes one second or five for your Pilum of Flame to reach it's target.
-K!
I had just been working all of this out, quoting examples from the book, and when I came to post, someone else had done something similar already 
After re-reading the MuVi guidelines I was deciding whether it was a superficial change or the significant change. I couldn't decide whether the "Target" it was changing was "Ind > Group", or "This guy > that guy". But I'm happy with the spell as it has been re-written.
I also agree that the original penetration should be used, not the defending penetration. As for if it needs to remain form specific, that I am unsure of. Wizards Fork is not form specific, so this spell probably does not need to be either.
As for people not having good vim scores, if you were expecting to be fighting other magi (hunting down marched magi, wizard war, Hoplite) this would give you an incentive to study Vim.
(It finally makes Vim useful!!!)
If one is indulging in Wizard's Wars ,
investing Mirror of Redirection and boosting the penetration as high as possible looks very worthwhile.
A variation of the spell with Target : Boundary , around the Covenant may also be worth the Vis cost.
Unfortunately it has to be cast on the spell as it is being cast. As the other spell is outside of the magi's Parma, we can effect the spell itself, but we have to identify the spell, then cast our MuVi spell at it. Unless we understand how their spell works, we cannot effect it.
Hmm, instead of having it based on the form, we could limit it to having to know the spell they are casting. You cannot effect a Pilum of Fire spell unless you know how to cast one yourself?
I'm glad that these things have been useful.
To expand a bit, I think altering penetration should be possible and should be its own effect.
Brutus is correct.
Well, I think Hermetic magi know enough about how any Hermetic effect works to affect it. Knowing the exact spell seems harsh, as there are so many infinite spells out there.
My objection to the form issue is just that the rules are especially vague in the MuVi section about it. In my game, I'd say toss it--someone else might want to be harsher and say keep it. But I may be biased: my current wizard is a MuVi specialist so I see all these problems up close and personal. Creating the same spell 10 times every time to be able to help your friends' formulaic spells (and mastering all 10 forms to be able to fast cast defenses against that once-per-campaign enemy) is just... lame in a section that already has so many restrictions.
Final version, I have altered the wording slightly to make its usage more obvious.
Mirror of Redirection
MuVi (General)
Duration: Momentary
Target: Individual
Range: Voice
Targeted
This spell allows a magus to alter the target that another spell is being aimed at. The target spell must have a range greater than Personal and can be redirected at any valid target. The mirror spell must have equal or greater Penetration than the target spell to effect it. The spell cannot be cast on Spontaneous spells and will effect spells up to (level + 1 magnitude) of this MuVi spell.
(Significantly change a spell, base effect, +2 voice)