MR and the essential nature

Salve sodales!

I need a clarification regarding a case brought by a player.

A person under a Cloak of Black Feathers (MuCo(An)30) spell does not change its essential nature and therefore continues to be effected by Corpus and Mentem spells, this is the canon and no problem here.

Well, what happens if I transform the air around me in a rock and hurl it at a target affected by a Terram ward?

The ward deflect the rock or is bypassed because of its "air essential nature"?

Is a spell like Wielding the Invisible Sling ok for hurl this rock or an Auram requisite is needed?

Thank you all for the help!

PS: sorry for the (very) poor english

He will be warded against.

Because its essential nature is air, but its assumed nature is rock, henceforth, he is subject to both. As is a magus in animal affected by mentem, corpus and animal (for his body part)

The ward will stop air transformed into rock , if it fails to penetrate.
If the penetration value of the ward is higher than that of the MuAu to Te spell , it will fail to stop it.
(at least this is my current understanding , erroneous as it may well be)

Well wards ward "matter" or "might".

Matter has no MR and a spell penetration is not appropriate (only if you conflict directly with the spell with MuVi is it used). But it can be a good house rule, in the case of magical animated matter to use penetration of wards vs penetration of spell moving material warded against.

VS might, this is easier: penetration of the ward vs might of the being.

AND whatever spell is used to throw it also penetrates.

Hey, Ravenscroft, I think this is actually erroneous... My understanding is that a ward's penetration only matters if the thing being affected by the ward has Magic Resistance (just like other spells). So if you make a Terram ward against rock, it will keep out all rock, even magically transformed rock. If a creature made out of rock and possessing a Magic Might were to try to walk through that ward, though, its Magic Resistance is what the ward has to penetrate. Consider a Corpus ward designed to keep people out of a circle: people without Magic Resistance can't enter the circle, but magi can as long as the ward doesn't penetrate. This gets confusing when you have a ward that is specifically designed to ward against creatures with Might, because the ward both needs to be a high enough level to affect a creature with that much Might, and also needs to penetrate that creature's resistance. But "ward against substance" wards don't care that the substance is produced with magic or not.

Thank you, now is all clear.

Is that really true? I went back and re-read the section on magical wards, and while it doesn't explicitly say that the ward does (or does not) need to penetrate the creature's might in order to effect it, it certainly seems to imply to me that the creature's magic resistance is already considered in determining the level of the spell...

This was clarified in HoH:Societates. Wards, like other spells, need to penetrate Magic Resistance to have their effect. By the rules, Aegis should also have to penetrate, though there are good justifications for it being special if you don't want to have to do that in your game. And of course you can modify how wards are supposed to work if you don't like that they have to penetrate. A lot of players do. I think wards are overpowered without requiring penetration, but that's based on my experience playing a min-maxed ward specialist...

That really depends on how how you define wards in hermetic theory, which of course the rulebook doesn't really do. Some spells are named as wards, so are some guidelines but wards themselves are only loosely defined as 'Rego spells ... which protect the target from things of the appropriate Form'.

So you can pick a fixed level effect that keeps things out ( Rego Animal or Mentem to suggest it does not cross, for instance) and build it into a fixed level ward that keeps anything out as long as it penetrates. Much more effective at high level, perfectly legal under the RAW, and why bother with general level wards that scale so badly and which you must relearn at higher levels as you grow in power ?