[Icelander]

The concept of the magical threshold around homes, which supernatural horrors cannot cross, is popular in modern urban fantasy such as the Dresden Files. It is written up in the article 'Safe as Houses' in Pyramid #3-58 p. 4-10.

In the Dresden Files specifically, thresholds have more and greater effects than is assumed in the article, however. I'm looking for ways to implement them in a campaign using the Ritual Path Magic system.*

Threshold Effects on Magic Use by the Uninvited
Not only do vampires and other supernatural horrors have extreme difficulty crossing thresholds uninvited, particularly those around a family home, but even mortal ritual magicians have to be careful of them. Anyone who crosses a threshold uninvited is very limited in what he can do with magic inside the dwelling, to the point that even powerful wizards can generally be assumed to be magically helpless while inside a threshold.

I can easily give a penalty to any use of magic within a threshold. I'd want a number derived from the Threshold Rating, probably a pretty severe one. Based on the Dresden Files, I'd want -4 penalty for anything with a 'home' threshold at all, i.e. Threshold Rating 10+, and an additional penalty per point of Threshold Rating above 10.

But what GURPS trait is this? I don't like being affected by thresholds as a 0 point Feature, because it's a pretty significant limitation for various supernatural beings and I want to have the option of having supernatural creatures that do not care about thresholds.

Also, being able to enter dwellings uninvited, but only at the cost of suffering a massive penalty to magic use doesn't sound like a Disadvantage, it sounds like a Limitation on traits like Ritual Path Magery and Ritual Adept. But how much of a one?

Wards and Thresholds
In the Dresden Files, establishing protective wards around a home with a good threshold is much, much easier than it is to establish wards around an office or a rented apartment. Those wards will also be much more powerful.

How do I best represent this in game terms?

Note that the rule on p. 7 in the article doesn't seem to be what I am looking for. Why would any mage elect to use his threshold as a 'matrix' to cast protective or defensive spells if the effects are to make their skill levels lower? The formula given, (skill + Threshold Rating) / 2, means that for most wizards with skill 12+, using this in a normal home (TR 10) means a net reduction in skill.

Home-field Advantage
Harry Dresden specifically discusses how wizards can create enchanted tools that function within the confines of their threshold, but not outside of it. These are much easier to make than tools that they can take anywhere.

Any suggestions on how to reflect this in game terms?

*If it matters, I'm using Mandatory and Significant Modifiers (Thaumatology p. 82) with energy-accumulating Ritual Path Magic. Ritual Adept (Connection) is unavailable in the campaign and Ritual Adept (Time) is generally not available to mortals (certainly not to PCs). I'm also using the article 'Ritual Path Magic Specialists' from Pyramid #3-66 p. 16-20 pretty heavily. Finally, the campaign world is almost indistinguishable from our real Earth, in this setting mostly Very Low Mana, with a few ley lines, places of power and ritually-significant times where the mana level is higher (often much higher for certain paths, traditions or rituals).