Hello
In the rules, ritual always use stress die
In the rules, mastery in non stress situation cancel the roll for botch checking.
Say i cast an aegis in a magical aura of 5 with 15 pawns of Vim.
I have the aegis ReVi 75 with 1 mastery and a familiar with 5 gold cord.
How many botch dies should i cast if i roll a 0?
Tricky. You get no botch dice for the Aura, because it isn't "alien" to you, and Spell Mastery would seem to remove the botch dice for Vis when the magi is "relaxed". So given the information given, zero.
Yup, no botch dice... until the covenant enemy decides to interrupt the casting... 
Chris
That brings up a question I’ve always meant to ask. Can you practice a ritual spell, for the purpose of mastery, without using vis.
I would say, yes. You can practice without using vis.
It's not mentioned specifically ITR, but since you can master a Ritual spell, and "practice" assumes casting many, many times, and vis (by the RAW) is a rare and precious commodity - if I were SG, I'd have to rule that it does not use vis. Prohibitively expensive otherwise, even the smallest ritual.
But that's just a ruling, not a rule. 
Thanks for remarks.. As almost always, my IRL saga is different ^^ because magic aura add botche dices, and mastery only take away the "vis" botch dies...
As you can put mastery xp after adventure without having actually cast the spell, i'd rule the opposite: you can master without actually casting the spell: you are just theorizing HOW it must be done.
Interesting question. I'm inclined to go the other way from Richard and Cuchulainshound - I'd say you can't practice a ritual spell without burning through a whole mess of vis. My view is that rituals are pretty bit magical events, hence the long casting time, esoteric requirements (as represented by your Artes Liberales and Philosophiae scores) and need for vis. If you're not using vis to power it, you're not really casting the spell - just going through the motions. That can teach you all about going through the motions, but nothing about channeling that kind of power and bending it to your will.
My view is that mastery of rituals in the OOH has mainly come from story events in the past, and is transmitted either from teacher to student or through (rare and sought-after) books.
For what it worths, a bonisagus in my saga was thinking about writing a tractatus about mastery in aegis. I dissuaded him (because IRL saga masterizing a ritual don't worth it) so i think books must exist in more canon Saga.
I have a related question: for spells with general guideline, a book about a spell fixed at a general guideline to level X (sample: Aegis 30 mastery quality 14) can be used for exact paramaters (aegis 30) or for all spells using the guideline, which level don't matter (aegis 10 or 25 or 70) ?
I seem to be inclined towards the "yes you can practice ritual spells for mastery" side. First, of course, it's the conclusion I hope to reach, because of game-mechanical outcomes. But also, it seems like there are plenty of things to practice for ritual spells - the "esoteric requirements", the complexity of setting up the ritual components, and so on - and that spending three months going over those details would legitimately increase the chances of succeeding and decrease the chances of a major screw-up. In other words, there's a lot to practice besides actually channeling the magic.
Also, I imagine that a ritual spell cast without vis still sort of works, but just not permanently. For example, if I cast that ritual spell that creates a ton of silver but don't use vis, then I imagine the silver does indeed come into existence, but only for long enough to deliver a silent argent "neener" before disappearing a moment later. Still, I could practice that all I wanted and presumably get some benefit from the experience.
It doesn't say that at all. Even when it makes a specific statement about vis botch dice, it says the same about auras. You're welcome to change this, but that's how the ArM5 rules read.
No, it doesn't. By that reasoning the rules say you can only learn a language where it is spoken natively. Rather, if you practice Spell Mastery that way you get a quality of 5. There's nothing saying you can't practice another way with a quality of 4 just as there's nothing saying you couldn't practice a language outside of where it's natively spoken. You could rule that that is the only way to practice Spell Mastery, but RAW makes no such restriction.
I find the 1st rank of Spell Mastery exceedingly useful with rituals because it offers so much protection against botches since rituals are usually cast in safe situations. I love Stalwart Casting when I can get access to it.
Normally they are separate. However, there is a Spell Mastery option that lets you use one score for all versions of a general spell. It's in HoH:TL in the Mercere section.
Chris
For the little question about mastery, i read in the Lion & Lily book that the tribunal has a book from Notatus about "mastery of aegis of the heart" and don't specify any level. BUT it also states, just after that, "there is a copy for a XXX level aegis". So we could interpret it in 3 ways:
Other thoughts from you about this question?
What's the point in the Spell Mastery option to combine the Spell Mastery for different levels of a general spell (HoH:TL Mercere section) if they're already combined?
Chris
I don't think it so simple.
Let's say A has a
When he learns a book, the book provides only source xp for one spell's mastery. Let's say he puts 15 xp in mastery 2 for the aegis 20.
The mastery which you speak about , IIRC, is useful to use the mastery 2 (for aegis 20) when casting aegis 30.
I agree, when reading this book he chooses which of his two Aegis - which are treated as two completely different spells - he wishes to master. I don't know if he can split the exp between the two. In theory, since the spells are the same, just at different levels.
If he gets Mastery 2 for the lvl 20 version, lets say he chooses Stalwart Casting and Adaptive Casting as his two mastery options. He can now cast the other version known, using his Mastery score as a bonus (and to reduce botch dice) as well as use Stalwart Casting.
He could start to master the lvl 30 spell as well, as a separate endeavour. He can't read the same book again I think, otherwise it would be abusive and an exploit*. If he attains Mastery 1 and chooses Learn from Mistakes, he gets exp when he Botches this version of the spell. Because the lvl 20 version has Adaptive Casting, both versions gain the benefit of Stalwart Casting.
Adaptive Casting only works one way, not both - IMHO. Meaning, the spell/version with which you have Adaptive Casting can 'transfer' other mastery abilities to the opther versions. The other versions may be mastered for other effects, but cannot transfor the use of these abilities. Unless they also have Adaptiive Casting.
') The potential abuse is not apparent with Ritual spells like Aegis, since they are naturally limited in not only which Mastery options are usable, but also the number of times used, due to the vis cost.
But consider instead a non-Ritual, general spell - like Demon's Eternal Oblivion.
A Magus might benefit from knowing this at several different levels, the higher level the more might stripped, but also lower Penetration. Let's say he knows it at 5, 10, 15, 20, 25, and 30.
He gets acces to a Summa in DEO Mastery by a good author with Level 2 Quality 15, not impossible since the Q can be raised by dumping Lvl.
He reads the book for one season, getting to Mastery 2 for the lvl 5 DEO, he chooses Adaptive Casting and Penetration. He has met the level cap, and thus should not be able to read the book further.
Second seaso he reads it again, this time for his lvl 10 DEO, so he get around the level cap. Again he reaches Mastery 2, and this time takes Adaptive Casting and Multicast.
Third and fourth time he again chooses Adaptive Casting as one and takes....Stalwart Casting (to avoid fatigue from the high level versions or if he suffers massive penalties) and....Fast Casting. And so on.
By the fifth season he has 5 different Mastery options - apart from Adaptive Casting - for his DEO, which he can use with all versions.I think that is an exploit. Had he tried to get 5 different Masteries with one single spell, that would have been expensive and taken much time.
IMHO when this book is first read, the magus chooses which spell is is Mastering, and from now on this book is labeled 'already read'. Like any other Summa read to the level cap. It is quite allright if he chooses to Master all 5 versions, but he needs other sources than the original summa.
And he can very well raisethem all to Mastery 2, with Adaptive Casting as the first for them all, to use the second mastery option for them all. It's just not as easy. Unless he finds 5 different magi each writing a Q5 Lvl 2 Summa on DEO Mastery.
Technically Aegis of the Hearth 20 and Aegis of the Hearth 30 are two entirely different spells using the same guideline outside of the power level, but they are two entirely different spells. Since they are two entirely different spells, you could write a book about mastering either one, but they aren't the same and aren't interchangeable.
Now I totally understand the idea of merging them, but they are technically two different abilities, unless you take that mastery ability.
The more confusing thing is what happens when you read one of those books and have that mastery ability which allows your mastery to apply to both. In that case I would allow both sources to be used, merging the spell masteries into one ability. Also, merging the masteries gets rid of the prior abuse that was pointed out. That's not really RAW, but it's pretty close and I would find it acceptable.
Chris
Restatement: my question come from the book in Lion and the lily (tribunall books) which specify "mastery for aegis" but no level AND THEN add "and this book has also a labtext for aegis 50". I'm just trying to understand what it means
Ah, I think the point is that the one book serves two purposes:
Lab text for Aegis of the Hearth 50
Tractatus for Spell Mastery for Aegis of the Hearth 50
Chris
Not quite. I would say:
Tractatus for Spell Mastery for Aegis of the Hearth
Lab text for Aegis of the Hearth 20*
*note that the actual passage from LatL reads:
"Research Notes on Aegis of the Hearth, by Notatus and others. Tractatus on Mastery in Aegis of the Hearth, Quality 11, plus a Lab Text of a 20th level version of the spell. Build Points: 15"
That is, as others have said, the tractatus can be used to provide 11 experience points in mastery for any level Aegis of the Hearth, not just a level 20 one. However, the book also includes a lab text for a 20th level Aegis. That was certainly the intent, as written.
It never occurred to me that a book on spell mastery for a general level spell might be specific to a particular level of that spell. Yes, if you know two versions of a general level spell, you need to develop the Mastery scores separately (else the Adapative Casting spell mastery would be useless); however, the two spells are basically the same apart from their power, so a book describing more optimised ways of casting the ritual should benefit either (not both) versions of the spell.
Mark
I agree. However, that disagrees with the ArM5 rules. I choose to follow the ArM5 rules instead of the example. There is a contradiction, so you can go either way.
Of course, if that argument applies, then they might as well share Mastery Abilities, too: understanding more optimized ways of casting the ritual should benefit either version of the spell. Yet that is not the case unless you take a special ability to allow it. My other issues with this are, for example:
The bold is mine to point out the issue.
Again, the bold is mine. Here we don't have logical disagreement (a conditional statement does not imply its converse), but we do have a show of intent. When stating all this the book could have made a statement to include various level of general spells, but it doesn't.
This combined with all the singular articles used on page 87 indicates that each individual spell has its own Mastery Ability and that a book is written for one Mastery Ability, not for several and take your pick.
Chris