As far as I know, the supernatural powers aren't affected by the local aura. I see nothing stating the contrary. Supernatural abilities are affected, as it is noted a numerous amount of time.
It may look a bit odd that a Faerie Greater (or Lesser, Focus, etc...) Power isn't boosted by a surrounding Faerie aura, neither countered by a powerful Dominion aura. Even devout priests are harmless in a cathedral (though they may have true Faith or Relics).
On the other hand there are a few Faerie power which are specifically designed to be castes in a foreign aura (like the one creating a comfortable home) and they do need not to be minored by said aura.
What do you think about it ? Could a Greater Benediction be a way to harness local Faerie aura as a Penetration or Finesse bonus ?
If you are saying "powers from creature are not affected by auras" I'd say it's wrong: see "penetration" text in the box on page 183, ArM5 core rule book: "for mystical creatures, there is no roll to use the ability (see page 191), so the aura modifies penetration directly."
I'd guess the term "ability" is not the right one, since on page 191, it speaks only about "creature powers" and not "creature abilities" and they have a "creature power penetration".
Anyway, it's my view, and in our saga, it's played in this way.
To my eye, reading the whole text on penetration from which ExarKun quoted, it seems quite clear that supernatural powers are affected by auras. Also, with regard to the author's intent, there is this quote from the same page; "The aura of a given place affects all supernatural activities that go on in that place" (emphasis is mine).
The same question comes up with enchanted items - if an enchanted item with penetration 0 is used in a cathedral with a divine aura of 5, do you allow it to work with a penetration of -15? If you answer yes, this makes enchanted items a little better than spellcasting, as spells fail if you fail a casting roll by more than 10 and cost you fatigue if you fail the casting roll.
Some people house rule than any effect with a penetration below minus ten fails, to stop faeries using powers and magi using items inside very strong divine or infernal auras.
IMHO the (otherwise fixed Penetration) of devices is modified by Aura. So I think the Penetration of -15 is possible. However, since I don't recall any MR being negative, it doesn't matter how low a Penetration total is, as long as it's below 0.
If you have MR 0 (like an apprentice with the shared PM (at 3 or less) of his parens) you till resist effects unless they have Penetration 1+. The negative Penetration effects won't affect you
But with no MR any effect which works will affect you. Spells can Botch, but devices always work. So it's better to have MR 0 than having none at all.
Exempli Gratia in canon material:
In TtA page 151 the device "Clasp of the Ambassador" has the effect "Aura of Equality" which has a Penetration of 32, specifically to affect even an Archbishop with the Commanding Presence power (RoP:D p42-43, MR 10) while inside a cathedral where the Dominion protects even further (Aura 5, RoP:D p10, aura mod -15 towards Magic). Even on holy days, where the aura might be 1 higher. But not a Cardinal (MR 20) or the Pope (MR 25)
Christian, I'm not sure what the power does, but if that archbishop has no MR, the aura wouldn't give him anything, so I guess he has some MR.
The NPC from tTA cannot know the value of it: he might have relics or whatever and the aura might add +value aura to that.
Even without the -15 from divine aura (which IMO is not applied), such item would need some penetration. 32 is on the side of a cautious item-builder.
If we consider the aura value aplies on penetration of item, then 32 is clearly not enough.
Commanding Presence is a Divine power which - among other things - grants Magic Resistance to the leaders of the church. An Archbishop gets MR10, a Cardinal MR 20, and the Pope gets MR25. For a magus dealing with church officials these numbers should be known.
A cathedral has Dominion aura 5, strongest at the altar (because of a relic placed there, as per The Church), and diminishes with distance (described in RoP:D p10). On Holy days the Dominion aura is +1. Total Aura on a Sunday is 6, penalty 3x6=18 towards magic effects.
Device has Penetration 32, modified for Aura (-18) this is 14.
Magic Resistance of the Archbishop is 10, so the device works.
Unless said Archbishop is holding a relic for further resistance. The creator of the device apparently did not anticipate this. RoP:D isn't very good with examples of relics, except for the really big ones. The head of John the Baptist is good for 5 Faith points or MR50. If minor relics can be assumed to be in the 1-3 range, then church officials below the office of Archbishop - who have no MR from a potenial Commanding presense - caught outside church on a weekday may be affected anyway.
I applaud your polite debate
Oh yes, I forgot that the Aura adds to the MR, as well as (perhaps) modifies the Penetration! Blast!
So forget about the bit about Sunday:
MR 10 +5 Aura = 15
Penetration is 32 -3x5= 17 (if modified at all)
I blame my poor memory, that device was designed a long time ago. Also these mechanics are rarely used in my games, it's usually magus versus magic creature in magic aura, or magus versus faerie in faerie aura.
I'll admit it's not totally clear from any direct source of rules. The device in TtA just fits my interpretation. And the phrasing is true, barring use on Sundays, when not holding a relic etc.
In the Realm interaction chart the text for Penetration mentions that Creature powers are modified by Aura. Spells are not, because the Casting Total already figures this in. So I interpreted a Creature Power to be similar to an effect in a device. No rules I know of directly states device Penetration is modified in this way. I just like if for balancing purposes, to ensure devices aren't all too powerful solutions for foreign auras. Perhaps this was the purpose, I just don't like it. Devices can get high Penetration easily enough, especially Charged Devices.
Anyway, if device Penetration is not modified by Aura, then the aforementioned Clasp of the Ambassador will work just fine, with room to spare. Unless it's Sunday and the Archbishop is carrying a Relic with 2+ Faith points. In which case the person using the device had better work on his or her timing.