The current Ars Magica mechanics allow for negative Penetration totals, since mundanes without any MR will get affected no matter the Penetration.
However, it also allows enchanted devices and supernatural creature powers to operate in suppressing situations like the Aegis of the Hearth. Which seems against the spirit of the game.
Hence I am brainstorming ideas for House Rules to limit the efficacy of negative Penetration totals. Perhaps the world has a certain intrinsic permanence that needs to be Penetrated for a supernatural effect to work, though the total is sufficiently below zero that most Magi have never encountered it. (eg the difference between ice freezing and Absolute Zero). The minimum required Penetration probably changes based on local supernatural Aura and effects like AotH.
I am personally a fan of Casting Tablets, and I see a vague similarity between Casting Tablets and enchanted devices - both have a completed spell effect ready to be released if the right trigger is used - in the case of Casting Tablets it is a Gifted person reading the "tablet", and in the process generate a Casting Total.
Taking the extreme case of a Casting Tablet, it is designed to work even if the user's Casting total is 30 or so below the required, the difference is made up in Fatigue from the user.
So perhaps that is a possible mechanism - if the enchanted device/supernatural creature in an AotH or suppressing Aura needs to pay Fatigue to "push through" with a negative Penetration total.
And enchanted devices can't spend Fatigue. So add Penetration to the enchanted Device when it is enchanted? Allow the first -10 to get through without Fatigue?
Any other ideas for limiting the efficacy of negative Penetration totals?
Addenda: I hadn't yet read the deceptively titled forum entry before I wrote this.
Something I have been considering to have as a house rule is that the Aegis would give everything protected by it a Magic Resistance of 0 (unless it already has higher MR) against all "foreign" effects.
That way items effects and powers activated inside the Aegis would need a certain amount of Penetration to affect things and beings protected by the Aegis.
Auras, on the other hand, are not specially designed to protect against magic, so there I don't think any special rule is needed to change the behavior of negative penetration.
Actually, it does feel "wrong" that a minor Infernal being can still freely use its powers on anyone without MR even in a very strong Divine Aura. So, I agree with the OP that there is something aesthetically displeasing with very negative Penetration totals, regardless of the source of suppression.
I like the direction Ivgreen suggests.
Any effect with negative Penetration simply fails, though any being that can spend Fatigue can "make up" any shortfall by spending 1 Fatigue level per 10 missing Penetration levels or fraction thereof (I would extend this, without limit, to Hermetic spellcasting).
Enchanted devices in general can't do it, and that's a feature rather than a bug - in my view ArM5, particularly as the line advanced, made Enchanted devices somewhat more powerful than they should be compared to spellcasting.
I'm not so sure I'd say it's against the spirit of the game for an item to still work considering DE's AotH explicitly says
many enchanted items do not need Penetration, and thus can be used within the Aegis without any problems.
And I'm not worried about the low-Might demon affecting mundanes inside because it can't get inside (unless someone brings it in), and from outside it can't penetrate.
There are specific ways to keep demons off temples, churches, mosques etc: see RoP:TI p.33f Demonic Weaknesses. Especially:
All demons are vulnerable to expressions of God’s power, whether these are the sacraments of a faith, prayers from devout followers, or the relics of saints.
Relics, as supremely holy items, are anathema to demons. They all have a Power called Scourging the Infernal, which means that they radiate an aura over an area with a radius equal to their Divine Might (i.e., their Faith score multiplied by ten) in feet.
Demons may only enter this aura if their Infernal Might exceeds the Divine Might, and even if they manage to be within this aura, they take a number of points of damage equal to the relic’s Divine Might each round, burnt by the holy fire which emanates from the relic (this damage may be soaked as normal).
Many altars contain relics, and some of these are true ones.
See also p.34 box Religious Limitations of Demons, and then on p.34:
Vulnerability: A certain substance (see Abhorrent Material for appropriate
examples) causes a Light Wound to the demon merely by touch. The more powerful the demon, the rarer this substance should be. Any weapons made of the material to which the demon is vulnerable inflict a Light Wound to the demon on a successful attack in addition to any damage inflicted as a result of the attack, even if the demon’s soak absorbs all the other damage. Light is a common Vulnerability of weak demons.
All demons have a Vulnerability to objects intrinsically part of Divine worship.
So weak demons are usually not found in places of worship.
Let's say, that a community believes their altar to contain true relics.
Take a look at Saint Nicholas the Pilgrim: Nicholas the Pilgrim - Wikipedia and the boon a Saint Nicholas was for Trani, since Bari had already Saint Nicholas of Myra as the patron of their cathedral and goal of important pilgrimages.
Contained within every consecrated Catholic altar are relics of saints, an ancient tradition that finds its roots in the very earliest days of the Church.
When Catholicism was still illegal under the Roman Empire, early members of the Faithful would meet in underground catacombs to escape harsh persecution. They would celebrate Mass on the stone slabs covering the tombs of martyrs in order to show proper reverence for the ultimate sacrifice they made for Christ.
When Emperor Constantine legalized Christianity, Mass no longer needed to be held underground out of necessity. Moving above ground, they didn’t leave behind the tradition of venerating the relics of saints. Churches were generally built over the remains of martyrs, but when not possible a first-class relic was placed within the altar and covered by a stone slab.
The Second Council of Nicaea would later decree that all churches were to have altars containing relics of saints. Eventually, the practice became ecclesiastical law, with early editions of the Roman Pontifical mandating altars to contain first-class relics from two separate saints, one of which a martyr.
Relics are physical remains of saints or objects that are believed to have a connection to the divine. They can be classified into three categories: first-class relics, which are parts of a saint’s body; second-class relics, which are items owned or used by a saint; and third-class relics, which are objects that have been touched to a first or second-class relic.
