I wonder how the Lesser immunity to Warping from constant Supernatural/mystic effects is more powerfull then Lesser Immunity Cold what is a RAW example of a minor Immunity at last if the person don't have the Gift.
Cold can actual kill you and is in effect for 1 saison per year in the region we play beside it also protects from some Perdo Ignem and Auram spells.
(I dont have MoH so I'm not sure how it is actual limited but that what I would expect that it's effects are)
To answer the question above, the Lesser Immunity to Cold in MoH only cover 'normal' cold (that is, exposure to winter temperatures). It specifically excludes extreme, magical cold (whereas the Greater Immunity to Fire in the core rules affects both mundane and magical fire).
The Longevity Ritual counts as a constant effect, and is ironically the single main cause of the Warping that finally carries off most elder magi. To me, that'd make it both common and deadly, and I'd agree with the requirement for it to be a Major Immunity for a normal magus; however, I believe this is for one of the ageless magi concepts, correct? You're quite right that it isn't going to be very common for him to pick up constant effect warping from any other source, so it sounds like (for him) that makes it fall under the "rare" clause and be a Lesser Immunity.
But extra botch dice are a blessing - they could send you into Twilight, where a worthy soul might find enlightenment within the glow of that which lies beyond this mortal veil...
Oh, sorry. Just getting in character for my Criamon. 
For a more serious answer, I'm not sure a Magic Aura actually increases botch dice for a regular magus. I could be totally off on this one but a quick look in the book at the auras section says the extra dice are for supernatural acts in a foreign aura. I think the same is true for Merinita Faerie Magic: they don't get botch die penalties for Magic or Faerie auras. I think you'd only be looking at extra botch dice for Infernal powers, or for Faerie stuff if you haven't learned Faerie Magic. Am I missing something specific to the experimentation rules?
In any case, a higher aura is always welcome by me. I think the covenant boons and hooks are already designed though, so I'm not sure if that one is up to us at this point.
I suppose a mortal would find enlightenment in Twilight. (Also getting into character.)
It does for laboratory experimentation and vis study.
This world is a trap, and if one is truly doomed to stay within it forever, a most terrible one. Perhaps your researches will some day show a path to freedom! Until then, I grieve for your burden and am grateful you have taken it on, for what you discover may some day release us all.
Ugh, found the reference! I missed that completely. I think I owe the experiment gods a few back botch-dice, if you know what I mean. 
No, this is for a Companion concept. The Companion immune to constant effect Warping has become effectively immortal. Immortality should cost a LOT more than one virtue slot.
Other sources would be items, or even just daily cast spells. This character suffers no penalty to be under constant effects of Gift of Bear's Fortitude, Aura of Rightful Authority, Preternatural Growth, Eyes of the Eagle, Eyes of the Bat, Wards vs. Weapons, Ward vs Heat and Flame, Ward vs Arrows, the list goes on and on. So not only is the character immortal, but once he accumulates some Hermetic wealth he can become effectively unstoppable. This is really abusable and quite overpowered, even as a Major Virtue.
Ok I see so I have to exclude the complet Hermetic magic even if one can avoid most of the longterm Warpings in using the enchanted items only when needed like when going for a fight (can be probably even done via Trigger).
So how about "no Warping from constant Goetic Arts effects" is this lesser or greater immunity or still to much in your mind?
Greater Immunity to Warping would actually be fairly reasonable when compared to some other major virtues. Notably: Ablation, Transformed Being, Blood of the Nephilim+Strong Angelic Heritage, or Demonic Blood+Demonic Powers (remove decrepitude FTW!). In addition there are a number of ways that while they do not grant immunity to warping do negate it taking you out eventually. Divine Assent, Faerie Calling, and Fatra Banya, all if managed correctly won't cripple or permanently remove the character. I'm sure there are other ways too.
The price to get Faerie calling can be paid with flaws.
So I can see why the GM might frown on "no warping" tricks, but the costs seems to be pretty close to around a major virtue.
Beside that without the GIft you accumulte Flaws but nevery get to Twilight or be removed from the game because of warping. What makes the effect not deadly compared to someone with the gift.
So yes you can keep your beloved Compenion / Grog alive for every if you can accept that (s)he get a new major flaw every vew decades.
As for Mundane characters you get a Minor Flaw at Warping Score 1 , a second at Warping Score 3, a Minor Virtue at Warping Score 5 and a new Major Flaw at each Warping score get past this.
And I only talk about a Mundane character.
It get even worse if a mage find a way to remove the Major Flaw(s) even if it cost the target some warping points.
Excluding Hermetic sources of Warping does allay much of my apprehension on the matter.
The only thing I'd caution against with Infernal characters is to be sure the infernal association is something that can be easily hidden. Since the Hermetic Oath forbids dealing with demons, that puts a target on the heads of Infernal characters from any member of the Order if you are revealed.
Actual right now its only to offset the Warping he would gain from the effect of the tainted Weapon.
So No there is no dealing with Infernal not even distant right now.
But yes sometime down the path this might change depending on the plans of our GM.
The Char only know that there is a second voice in his head now from the spirit he have slain what is strange to him.
He dont even know why this is so or that it was the effect of the weapon.
You could be a jerk and take Faerie Antipathy. That shuts down warping pretty quick. Anyway GM call.
Anyway, while strictly speaking dealing with demons is not a crime if a companion does it. Nor is using infernal abilities if there is no contact with a demon. A magus better have a good explanation for how they got them without demons. A companion OTOH isn't protected by the code, so will likely simply be killed out of hand if possible. (Of course the goetic arts are quite strong, so there is no guarantee the would be killer could manage.)
Actually my bigger concern is that this sword grants a fairly significant power and xp for just a minor virtue. Normally one would need to exceed the spirits might to bind it AND a major virtue.
If an average magus can't kill a companion, then that companion should actually be played as a mythic companion, a primary character. Just saying 
OK I actual didn't put a efford so fare in looking for a similiar hermetic spell what I just did now (HP 97) and found out that the level get to high so that it don't work out with this virtue.
Sorry about this, just thought it was a nice idea for storys as the spirit isn't actual controlled trough this magic.
Looks like I should come up with some other idea what to do with the 3-4 remaining virtue points.
Now unless you optimise your mage I think a 21 year mage will have big problems to win a live and death fight with a 21 year ald Companion Warrior.
I would even go so fare to say that given the gamesystem the one who manage to Suprise the other have a huge enough advance to decided over live and death even if both get older.
Are you thinking of a different game system maybe? This is certainly not the case in Ars Magica.
With Fastcasting chances are that the mage will get off at least one spell before a mundane (ANY mundane) can attack. No MR is a major problem. Even if a magus is particularly slow and can't manage to fastcast before suffering the first attack, unless a mundane can incapacitate a wizard in a single blow, the mage will probably kill the mundane. The mage only needs a single spell, and doesn't even have to worry about Penetration. Mentem wizard? Makes you fall asleep, cuts your throat. Corpus wizard? Disables you, then can take their time to kill you. Fire/Auram/Terram mage goes for the direct kill. Aquam mage fills your lungs with water or dessicates your body while you're still alive. Imaginem wizard, you can't hit him because you never know where he is, he has to get creative with you... maybe lures you into falling off a cliff by making it look like he's standing over there. Vim wizard has to be creative too, but maybe he uses MuVi to change a formulaic spell that he knows into a different formulaic spell that can do direct damage.
Anyway, no matter what a magus' specialization, they should be able to kill an adversary who has no MR using nothing more difficult than third magnitude spells.
@Ryu so you say that a non specialised mage just out of his aprenticeship can do this?
first fastcast gives you -10 then unless your mastered a spell its spontancasting what 1/2 the score you get and then they still get above 15 so having form+technique+sta>35 to be halfway reliable?!
GIven that apprenticeship usualy start at the age of 8 and most apprentice need to be teached Latin to 5 (75 exp) from 0, Artes Librares to 1 (5 exp) , Magic Theory to at last 3 (30 exp), all forms to 1 (10exp) to follow the default pattern given in apprentice and the core rule book I doubt your average apprentice end with the arts high enough with the 240 exp you have to spend on all this.
Your talking about the optimised Mages players make with the rules but not what should actual be the average mage out of his gauntlet.
You don't NEED the fastcast to do it, it just helps.
If you have ANY magus character that can't kill a person without any MR, you're not being creative enough.
I didn't say that a mage can't kill a compenion char thats fare from it I just sayed they have problems as even the age 15 Berserker template from the core book have the ability to kill a mage in one hit.
(We haved this in our group that a unprepared Arcmage was hit by a good grog warrior and was deadly wounded with just 1 hit)
Thats why i.e. mages shouldn't anger theyr own guard grogs to much or they could come to bad ideas for your mage at times the mage never expect a attack.
Treating one's own underlings poorly is just bad form.