My SG came up with a very good question - it has been over 100 years since the schism war, how would anyone recognize deidne magic if they saw it? All those who saw it first hand have probably passed on, all 'younger' people have to go on are whatever stories and accounts they left.
There are lots of mystery cults and special abilities spread around the order - who can say for certain that anything is/isn't Deidne?
My personal take is that Diedne magic at the very least is decidedly non-latin, and therefore... if/when you use it, I think that much should be obvious. Which is enough to get you the 'hedge wizard' reputation/stigma even if it doesn't by itself give away the secret of what kind of non-latin.
As to who could actually recognize it for what it is, the first thing that springs to mind is the 'lineage of Pralix' from the House Misc. writeup. Mmm. I'm sure there are others. Particularly of the Tremere, Tytalus, and Flambeau bent who had masters who were veterans. It isn't a large leap in logic to assume such veterans would teach their apprentices what they know about the magical practice of the enemy.
Hmm. But as for putting a hard fix on it? No... I agree it is a grey area. On the one hand I think it is illogical to assume that it wouldn't be something people could recognize. On the other I get the impression a lot of the play of people with that lineage would disappear overnight if the risk of being discovered became more real / pronounced. But is that such a bad thing? I mean... you have to take that dark secret flaw for a reason.
Plus the possibility of people stuck in regios where time moves at a different rate, if at all...not to mention magi on either side of the schism war who became faeries or something. There is also the possibility of ghosts that can be contacted who either were or fought Diedne. There is also the possibility of non-magi who were around at the time of the Schism war (magic creatures, faeries, angels, demons). Some of these things may have even fought in the Schism war on one side or another.
My personal take is that Diedne magic is not much different from mainstream Hermetic magic. I would think that an effect produced by a Diedne would look to Hermetic InVi investiagation much the same as any other Hermetic effect. Just like a Pilum of Fire cast by a Bjornaer doesn't have anything Bjornaer-like about it, which can identify the caster as a Bjornaer. I think that a Diedne would only be identifiable if he used some unusual effect known to be associated with Diedne --- I don't think there is any such thing in canon, although there are hints --- and the hidden Diedne would presumably be very cautious about using such effects. A Diedne (who was a Schism war relic) could also be identified by his casting sigil; but that doesn't seem to be a risk that any newly minted Diedne run.
I don't see any reason why that should identify the caster as Diedne? They are just Hermetic magi. They probably have a non-latin cultic language (like Bjornear have Gothic) but that doesn't imply that they cast spells in anything other than latin like all other Hermetics.
If Diedne had to do something uniquely weird to cast spells (like Verditus need to use casting tools), that would identify them...and if that's what they do in your saga, that's fine.
However, I think it works better if they don't do anything unique, because it is then possible for Diedne to hide amongst other Hermetics (which fuels Hermetic hysteria about the Diedne). It seems (to me) to be totally implausible if Diedne do have unique casting methods for any but the most naive of magi who witness the casting to be in any doubt that they are watching a Diedne. So, this basically removes any chance of telling stories about Diedne hidden within the Order (you can still tell stories about Diedne hidden in regios or elsewhere remote from the Order, of course). Also, if there is nothing inherently unique about Diedne casting, it means it is worthwhile for characters to go looking for ruined Diedne covenants --- as any lab texts, books etc that they find are useful --- which is good, because there is lots of opportunity for story there (including being suspected of being Diedne).
Actually, thinking about it... Didn't Diedne introduce spontaneous casting to hermetic magic in the first place? Each founder had a contribution, and I was under the impression that spont magic was Diedne's...
In which case, all spontaneous casting looks Diedne-ish, so... Well. You could guess that someone is Diedne if they relied a lot on spontaneous magic, but only in the same way that you could guess someone was Flambeau by watching them throw Ignem spells about.
Precisely. Diedne were Hermetic magi, and the Schism war only occured relatively recently, so virtually everything a Diedne magus does should look like normal Hermetic magic. Although it is likely that they have some special effects that are unique to their Mystery Initiations, which (if they are witnessed) could be identified as Diedne by someone with suitable Order of Hermes Lore and/or Diedne Lore.
Having said all that, it is always possible that the Diedne Mysteries render all their spell casting weird, but that is only one option.
We always took it to mean that IF a Diedne used the bonus effect that the way it was done was detectable (ie the way it was invoked, the movements etc...) Otherwise, how would they ever have been found?
The war started around the turn of the milenium, over 200 years ago. It doesn't say when the war ended, but even saying it lasted until 1070 - that's still 150 years. That's quite a time for a magus to live after taking part in the war.
Quote from core book page 10
"Since the schism war, the order has existed in a state of relative peace and cooperation. However, by 1220 the last magi who can remember the war have died or passed into final twilight, memories of that terrible time are fading."
As for the ghosts, a ghost could describe what the Diedne did, the magics they used - but it would be hard, just from those stories to spot it in action.
(Of course we know that the Tremere have a few pet Diedne kept under wraps, being closely studied to find out how their magics work - and so that they can recognize it if it ever resurfaces)
The core characteristic of Diedne magic is human sacrifice.
Obitus of Tremere
Given that Tremere haunted them down for human-sacrifice for their gods, I think it is fair to conjecture that it is at least possible that Diedne magic, when used to its full effect, involves religous motiffs and is thus plainly different from vanilla Hermetic magic. This would only be apparent in their "mystery" magic, though - just like an Hermetic magician using Names of Power, perhaps.
You could actually follow a similar line of thought for most virtues (or even flaws) that affect spell casting, to the point that pretty much each magus has his own unique way of casting spells, which is acknowledged in the setting, if only through the presence of sigils during spell casting and the fact that you essentially re-invent spells. Unless you are a Pralician trained in that kind of things or had the same parens, you probably wouldn't be able to recognize Diedne magic off-hand, nor probably even be able to tell that there is something weird to the other's magic -- everyone's magic is at least somewhat weird.
Now, if your magus also has the Exotic Casting virtue (from Societates), then yeah there probably are enough specific references to Diedne-associated motifs that a Per + Hermes Lore roll could give rise to serious suspicion (even though you cannot tell what kind of spell he's casting anymore).
I'm not going to make weak jokes, I will use Urien's most recent post as a short-term AC to him. That will give me enough penetration on my lvl 15 InCo spell to gain his date of birth. Now all I need to do is capture the electrons that came from his PC and I will have enough ACs and modifiers to re-write his memories of this entire affair.
I can't remember the name of the spell in A & A, but it was wickedly simple to get someone's birthday.