Reading through TM:RE something struck me. There are several paths for a mage to become immortal described in that book - and a few other paths in other books.
The one thing they all have in common is that to become immortal the mage needs to transform himself/herself into a supernatural being with a Might score, associated with one of the Realms.
Does this not break the Limit of Essential Nature?
ArM5 p79 states that "All human beings are essentially human; mortal creatures [...]"
After following one of the paths to immortality a mage is no longer really human, and obviously not a mortal creature any more, so it seems to me they must have somehow changed their Essential Nature.
It was my understanding that the two greater limits - Limit of the Divine, and the Limit of Essential Nature - applies to all sorts of magic, not just hermetic magic. So Mystery initiations should not be able to break that limit either - especially not Hermetic Mysterie Virtues, like those in TM:RE
This unlike the Lesser Limits which can sometimes be bent or broken with various non-hermetic traditions.
I haven't seen any examples of magic breaking the Limit of the Divine, but it is fairly common for Mystery initiations to break of the Limit of Essential Nature. Otherwise, any ongoing magical effect would necessarily incur warping for long-term magic. Initiations can cause permanent changes to the subject without requiring an ongoing spell, in violation of the Limit. No doubt some Bonisagus magi are studying the topic, though are likely hindered by the problem that Mystery cultists don't like to talking to Bonisagi researchers.
The limit of Essential Nature is a very interesting case, precisely because it is at once both so hard and so easy to violate.
consider this: A magus born in the year 1195 and now it is 1220, thus they are essentially 25 years old. Next year, in 1221, they will be essentially 26. Without doing anything their essential nature has changed!
A PeCo (Or CrCo) spell can change the age of a person in all ways but the essential one. This change is unnatural and permanent it only differs from the essential age in that changes wrought by magic can be undone by magic.
Most people are essentially two-armed, but an unlucky pilum of fire or a PeCo spell to dis-arm could still remove a persons quite permanently, yet a muto spell would have to expire.
Consider also CrVi spells that add warping points to a person, these warping points are permanent and give flaws (and eventually virtues) that are, maybe, part of the targets essential nature?
Then there are even stranger things, like Muto botches that can indeed permanently transform a person. But for something less extreme consider that muto spells can have permanent effects on their subjects as described in the sidebar on MuCo spells (corebook p. 132): "Transformations can have subtle (or sometimes dramatic) permanent effects. [...] When transformed back into a human, the target might temporarily or permanently retain some feature of the other shape."
To me it seems that the limit of essential nature is more about what hermetic magic can do quickly and reliably.
Magic seems to be perfectly capable of overriding essential nature some of the time but usually in ways that are hard to control or predict.
Not so. See ArM5 p.79: "the limits of Hermetic Magic are described below. Most Hermetic theorists believe, that there are only two fundamental limits, the Limit of the Divine and the Limit of Essential Nature, <...>".
See also TMRE p.4 What are the Mysteries? to verify, that TMRE Mysteries do not need to lead to "Hermetic Mystery Virtues". The closer a Mystery Virtue is to Hermetic magic, the easier to make it work with that magic. But stuff like (TMRE p.82ff) Ascendancy to the Hall of Heroes is definitely not Hermetic.
Hedge Magic Revised states that Hedge traditions are usually bound by the limits of magic that hermetic face, with one or two lesser limits being broken at most.
Of course, as people have pointed out, there are many exceptions to this. The first Hedge (in terms of non-hermetic) traditions that regularly flout some limits are the Sahirs, who are able to turn the clock back on their age with their elixirs and restore fatigue levels. Similarly, their is a mentioned sahir who somehow turned himself into a Jinn (and likely became immortal as a result of acquiring might).
There is also the Cult of Dedun mystery which ultimately makes a Gifted initiate immortal.
So yes, mysteries break the Limit of Essential Nature and it is kosher to do so. There is precedent in at least four books that one can break the limit of Essential Nature with mysteries.
No, that is not a change in their essential nature. It is part of the essential nature of humans to age, so someone becoming older does not change their essential nature.
There are lots of possible changes that can happen to a person that does not change their essential nature.
A person having a hand cut off, for example, does not necessarily change their essential nature of having two hands. A person with a missing hand, where the missing hand is not part of their essential nature, can have the missing hand restored with magic. A person whose essential nature is to only have one hand, can not have their missing hand restored with magic.
Looking through the mysteries in TM:RE and HoH:MC, apart from the immortality mysteries, there are very few mysteries that can be considered to change someones essential nature, and none that clearly do so.
"two fundamental limits", yes. I have no problems at all with Mysteries or various non-Hermetic traditions breaking any of the lesser limits - it is pretty well established that at least some of them are merely flaws in Hermetic Theory.
The two big ones - Limit of Essential Nature and Limit of the Divine - are supposed to be far more fundamental limits though, else there would be no reason to separate them from the lesser limits.
Blockquote
A person having a hand cut off, for example, does not necessarily change their essential nature of having two hands. A person with a missing hand, where the missing hand is not part of their essential nature, can have the missing hand restored with magic. A person whose essential nature is to only have one hand, can not have their missing hand restored with magic.
The thing is that one can become the other which means that it is possible for someones essential nature to change over time.
By Hermetic theorists! "Most Hermetic theorists believe, that there are only two fundamental limits, the Limit of the Divine and the Limit of Essential Nature, <...>"
Theorists of other magical traditions, even followers of Quendalon's Arcadian Mysteries (HoH:MC p.92ff) see this quite differently.
If other traditions can break the Limit of Essential Nature, then the Hermetic theorists in question are wrong, and that limit is just one of the lesser limits, reflecting a flaw in Bonisagus' theories.
Can it? Apart from a divine miracle, or a handful of the Mysteries (which I question precisely because they clearly break the Limit of Essential Nature), what other ways are there for someone to change their essential nature?
Mystery ordeals can inflict a flaw which alters the essential nature of the initiate. Someone who loses a hand in a mystery ordeal has that injury become part of their essential nature. For that matter mystery initiations in general can also grant virtues which alter the essential nature. Anyone can invent a mystery cult lore and devise self-initiations to grant pretty much any virtue or flaw (not without a lot of difficulty, mind you). In a sense mystery initiations are complex rituals which alter the essential nature.
Outside of mysteries exposure to the magic realm can. For mundanes warping causes changes to their essential nature as they gain virtues and flaws. Even for the gifted, if they spend time in the magic realm and accrue vis in their body they can be transformed and alter their essential nature with new virtues and flaws.
Likewise lab accidents and twilight can. Twilight scars are a part of the nature of the magus and can't be removed. Lab accidents can result in permanent alterations to a person - the most famous example being Mercere the Founder losing his gift in a lab accident.
There are quite a few ways that what hermetic magic calls the "essential nature" can be altered, and magi do it pretty regularly - just using ordeals rather than hermetic magic to do it.
That pointed me in the right direction.
TM:RE p11 does indeed state that Mystery Initiations do affect your essential nature, making any flaws or virtues obtained by the initiation a part of your essential nature.
So alright, there are a bunch of ways to change ones essential nature.
Still, for the various ways of becoming immortal listed, it is not Mystery Initiations themselves which change your essential nature into that of an immortal, but rather the rituals/methods you get access to through the initiation. But I suppose that is a rather minor quibble, best handled by ignoring it.
True, and I do wonder if virtues like Transformed (Being) are valid for mystery initiation? There isn't any limits placed, mechanically, on what virtues can be initiated via mystery but in terms of the internal logic of the setting and narratively does it make sense to be able to essentially have an ordeal which grants immortality as a magic being? I'm on the fence myself.
As an aside I think there is one way to definitely become immortal without mysteries. If you spend enough time in the magic realm to accrue enough vis to gain a major virtue you can become a Transformed Human, gain a might score, and become immortal. Easier said than done to get into the magic realm but a lot less effort in some ways than things like the Becoming or Ascension to the Hall of Heroes. I suppose the downside would be that what sort of Transformed being you become would be dictated by the part of the magic realm you spend time in. You would have less control than with the more "established" methods of becoming a supernatural being.
Mystery Self-Initiation into transformed beings is how the spider is able to take over new bodies and keep their arts that he and the new body posses.
Don't know if the scripts themselves are written anywhere in the book with him but I think that's how it's explained.
I don't see why one couldn't have a mystery initiation which grants you immortality directly, but I do suspect that an Ordeal by itself would not suffice for the initiation script. You would probably need a lengthy and complicated Quest as well, and likely some major Sacrifice also.