All this sounds very similar to Graeco-Roman conceptions of Necromancy.
Now where's my write up for Guorna the Fetid gone...
Jarkman
Yes, but precious little is said about how that Holy magus goes about learning those abilities. I think I'm going to create a new thread about that question.
Oh, I'm sure it is. Even some of the flaws in the book are definitely tempting. Fear my Corrupt Pilum of Fire.
I must have missed this when you asked it, sorry.
Infernal powers can be learned as Supernatural Abilities by anyone who has been somehow exposed to the infernal, like with an infernal Warping Point. You just have to get the experience from a source with Quality 5 or better (to ensure you get five experience points). They don't have to have the Gift or True Faith, like people who want to learn magical or divine Supernatural Abilities. You do have to subtract other Supernatural Ability scores, but if you don't have any it isn't much of a penalty. Magi can't easily do this, but companions sure can.
Anyone with True Faith can learn divine Supernatural Abilities by studying them from a teacher or some other source. There are no requirements besides exposure to the infernal to get the unholy ones. Magi have to subtract their Arts scores from their study totals, but not for the Favored Abilities of their tradition. Thus, a magus who joins the Witch-Hammers, for example, can easily learn the four powers. Other characters can join too, though. They're called Hermetic because they're most common among magi, not because only magi can belong. For example, the Daughters of Erictho are rarely magi. You can't learn Hermetic Virtues without The Gift, though, so Thessalians who aren't magi probably can't learn Chthonic Magic, and divine hedgies can't learn Holy Magic.
Note that since you don't subtract Favored Abilities from study totals and you don't have to subtract other Abilities when learning Favored Abilities, hedgies who don't have Arts can actually learn a few powers besides their favored ones. If they start young, an infernalist in a tradition like the Witch-Hammers might learn the other Goetic Arts in addition to Summoning and Ablating through play, or pick up Debauchery or an Ability like Sense Passions, Ceremony, or Corruption. I expect a powerful diabolist to have a lot more powers than just the favored ones of his tradition, because they are so ridiculously easy to learn. The hardest part is finding the teacher.
Wow. Hell really is pandering to the masses. Have they no decency? 
OK, hold on there. Does that mean that one can really belong to multiple traditions? It was my understanding that the Hermetic Arts essentially were the favored abilities of the Hermetic "tradition". Are Hermetics treated specially (wouldn't be the first time), or are you allowed something like one tradition from each Realm, meaning that you could end up, ignoring the flagrant incongruity of a Divine/Infernal combination, with a Luciferan Hermetic Ascetic who would have three sets of Favoured Abilities?
Could a Hermetic Magus join a Magic hedge tradition and benefit from their Favoured abilities? Are you allowed one tradition per Realm? One tradition from another Realm on top of your initial one?
I fully understand that the complete details should be in Hedge Magic once it gets written (they'd better be), but aside from different Realm affiliation and virtue requirements, we already have hedge "magic" traditions today. My mind is stuck in the "Hermetic Magic is a tradition, you're only allowed a single tradition" rut.
But could an experienced Gifted Damhadh-Duidsan be apprenticed in the Order of Hermes without suffering penalties to his study totals nor require a master with an outrageous Intellego Vim total to open the Arts?
If you create a character as a member of some tradition or other, which abilities do you have to buy a Virtue for? Those abilities you want to begin with a score in? That would make a rounded "apprenticeship" impossible, given that they're typically Major, and we are talking about Gifted characters (or whatever requirements for the other realms), not Mystic Companions. Or is the benefit just a starting score? e.g. Ex-Misc Damhadh-Duidsan who take Incantation get the free level 1, but those who pick Giant Blood don't, and both can buy up higher scores at character creation?
Word. It's not even that hard, since there are probably untold numbers of Demons just falling over themselves to offer a deal to anyone they overhear expressing a desire to do so. 
If I may offer a bit of reasoned advice, the deal is the same for both Hermetic and hedigies:
Learn an ability from Virtue at character creation (representing inborn talent, training from your mentor-this includes the special Ex Misc bonus- and oddball cases), no penalties.
Laern the "pure" ability with the Gift/True Faith or just exposure to the Infernal prerequisites, with cumulative learning penalties.
Join one tradition only in your lifetime, get to learn their own special set of four Favored Abilities. Set is the same for both Hermetic and non-Hermetic Supernatural Abilities, so Hermetics get a double-dipping; luckly bastards. No penalties.
Initiate a Mystery Virtue. No penalties.
No, the Hermetic Arts aren't favored abilities, they're opened through a special ritual the first season of apprenticeship. You could certainly have a character with four or more Supernatural Abilities before the ritual, but it makes the ritual harder, and it means your master has to have a high Intellego Vim total or it could fail completely.
One tradition in your lifetime. It's not always easy to determine when someone's joined a tradition, but basically when you learn something as a favored ability, you've joined the associated tradition. If the source you're learning it from doesn't have a tradition, you can't take it as a favored ability.
There are different traditions of Hermetic Magic, too. Mercurian, Faerie, Diedne, even things like Mutantes magic. Many of them have been converted into Hermetic Virtues, though, and once they become Hermetic they are rarely compatible with the previous tradition any more (Holy Magic and Chthonic Magic are two interesting exceptions).
Alas, no. He'd probably be stuck as a hedge wizard.
Any supernatural virtues that you start with, you have to take as Virtues. Ex Miscellanea get one for free, as you note. If a Damhan-Duidas takes Giant Blood as his free one, he can learn any of the other four during play, by seeking out others from his tradition and learning from them.
Yup. There's even a couple of examples of this in the book, like Aqrab the Conjurer (pages 58-59).
Great, thanks for the clarification. 
[size=150]Elations and Joyfull Celebrations.... [/size]
The postman was just here a sec ago - and now I've gotten my copies of Infernal and TMRE! Jubbbiiiiijae!
Here's a question about diabolists that it didn't occur to me to ask before:
What if diabolists do have The Gift? Could they just learn any of the Maleficia without needing to take the prerequisite Virtues? I had a delve through The Infernal but it didn't make the issue very clear, although I guessed from what it says in other books about The Gift that this would be the case.
From what I've read in other posts it sounds as if anyone can learn an Infernal Supernatural Ability so long as that character is willing to imperil his/her immortal soul. If I understand things correctly, a Gifted Infernalist has no real advantage here except, perhaps, the option of taking Hermetic Virtues...
According to the book the gifted diabolist would be able to use infernal range/duration/target.
By the way, I would allow it to use the infernal abilities with some hermetic magic. For instance an infernal spell using hermetic range/duration/target. Maybe even mixing hermetic arts and infernal abilities.
Indeed, but diabolists who don't have the Gift are created as Mythic Companions, with 21 Virtue points of which some must be used to gain the Virtues necessary to use the Maleficia. A non-magus with The Gift, however, can learn Supernatural Abilities without having to take the pre-requisite Virtues, and I was wondering if the same rule applied to the Maleficia.
The Gift allows characters to learn Supernatural Abilities aligned with the magic realm. To learn Supernatural Abilities aligned with the infernal realm, all they need is a Warping Point from an infernal source. These are separate. Since the Maleficia are infernal Arts, having The Gift doesn't make the character able to learn any of them, but experiencing the infernal does. I suppose there could be magic powers like Hex or Summoning that are tainted by the infernal but still magical, and they would need The Gift to learn those.
Also, I don't think this has been clarified anywhere, but it could be that when you subtract the value of existing Supernatural Abilities from your study total, you don't subtract the ones aligned with a different realm. For example, a magus has opened his Gift, so studying a new magic Ability has at least a -15 penalty, but if he decides to learn the infernal Maleficia, the first one's free. I like this idea because it means magi can dramatically change their magic and their powers when they become interested in one of the other realms.
EDIT: I don't have my books here, but I looked back at this thread and it looks like it must be stated somewhere that you do subtract Supernatural Ability scores from other realms unless you are joining a new tradition. Sorry for the wild and unfounded speculation based on my fallible memory! 
As you note later, this is utterly wrong.
The Gift allows a character to learn any Supernatural Ability. These rules are blind as to what realm a Supernatural Ability is aligned to, and some Supernatural Abilities may come in several flavors. When a Gifted character tries to learn a new Supernatural Ability the Source Quality is penalized by his existing Scores in Supernatural Abilities. All Supernatural Abilities penalize the Source Quality. It doesn't matter what realm they are from. The only time that Supernatural Abilities don't penalize the Source Quality is when the character has had his Gift Opened by a magical tradition, which makes some set of Supernatural Abilities "Favoured", which means that they don't penalize each other.
This is not the only way to learn Supernatural Abilities, obtaining the correct Virtues somehow (such as via a Mystery Cult) is the other main way. There are apparently other ways to learn infernal Supernatural Abilities in ROP:Infernal.
That's kind of what I was thinking as I was reading the information about Damhan-Allaidh and the Damhan-Duidsan in RoP: The Infernal. It made sense to me to assume that he at least had The Gift as members of his tradition could later become Hermetic magi, so I was guessing that Gifted diabolists would have some way of opening the arts comparable to that of magi.
The issue is merely that this does not seem to have been thought of at the time of writing ROP:Infernal. Or it was thought of and discarded for some reason.
However, your idea seems entirely sensible to me. This is approach taken in Hedge Magic: Revised. The hedge traditions described there each have ways of "Opening the Gift" to their tradition. You could perhaps use the examples there as an inspiration for diabolical "Openings of the Gift".
Well, I think saying what I wrote is "utterly" wrong is disingenuous... For one thing, The Gift most emphatically does not allow a character to learn any Supernatural Ability. It's clearly stated in ROP: The Divine on page 47 that only characters with True Faith can learn Divine Supernatural Abilities, and they learn them "just as characters with The Gift learn Magical Supernatural Abilities". And ROP: Faerie also specifies this ("This works exactly like Gifted characters learning Magic Abilities") on page 116. Actually, now that I look at it, ROP: The Infernal also specifies that The Gift allows "magical" Supernatural Abilities on page 126.
Other Supernatural Abilities do penalize the Study Total whenever a character studies them to learn them, that is true. But many of them are studied by characters who do not have The Gift, and in that case they obviously don't need to "open" The Gift to learn them. And of course, there's what happens when a magus joins a divine or infernal tradition. They immediately get four favored Supernatural Abilities associated with that realm that they can learn without penalty, without having to subtract their Arts scores or any other Supernatural Abilities from them. Faerie arguably works the same way (I think it should, and I wrote it), though it isn't stated explicitly.
This is a very puzzling accusation for you to make, as it seems to me ROP: Infernal makes quite clear that the Damhadh-Duidsan are a Hermetic tradition of infernalists. All of them have The Gift and have opened the Hermetic Arts, and they also have Favored Infernal Abilities. Of course, there might also be non-Hermetic versions of them as well, who don't have The Gift but still have the favored Abilities.
It apparently seemed unclear to Aurelius? I think he was asking whether there was non-Hermetic "Opening of the Gift" for these diabolic traditions. To the best of my knowledge ROP:Infernal does not explicitly state that there are diabolic equivalents of Opening the Arts for Gifted characters. True, it might be sort of implied that something like this happens --- as you say the character "gets 4 favoured Abilties" --- but this is not explicitly described as being the same thing as "Opening the Arts". Or maybe I was misreading what he was saying.
Also, Favoured Abilities has no meaning if a character doesn't have the Gift. You cannot learn a Supernatural Ability without the Gift.
No offense intended, but this all seems to contradict the core rulebook.
The core rules aren't as clear as they should be, which is why it's clarified in later books. The Gift allows you to learn Magic-associated supernatural Abilities, True Faith for Divine, one Infernal Warping point for Infernal, and a Sympathy score for Faerie (IIRC). The Gift is neither necessary nor helpful for supernatural Abilities associated with other realms.
Hmm, perhaps we're all talking about different things then. "Opening The Gift" isn't necessary for any supernatural realm except Magic. It's possible that joining a tradition for one of the other realms has a ceremony that is like Opening The Gift (i.e. "Opening the Faith"), but you don't have to have The Gift to learn Supernatural Abilities associated with those realms, so it's not really the same. It does work exactly as you say for the Magic realm of course, but that's because only characters with The Gift can learn Magic Supernatural Abilities.
Aurelius, can you clarify what you're asking? You want to know if Gifted diabolists can learn the Hermetic Arts, or are you maybe asking about some sort of Opening ceremony that gives them all four of their Favored Abilities?