It occurs to me, rego magic can make a person do anything they can naturally do anyway, correct?
If this is the case, can a Bjornaer be hit with a rego corpus spell to force them to transform into their heartbeast? Or a rego animal to make them change back?
If so, this highlights something about the nature of the heartbeast. If nothing else, if it CAN be forced by rego-corpus, it proves house Bjornaer right about it being a natural part of the magus.
Spells can only create effects for the guidelines that exist, IMO no such guideline exists. Also, I o no Bjornaer would bother researching this. Some other magi of other Houses might be interested, but probably wouldn't do the work.
Supposing such a guideline existed, I'd peg it as at least base 20, since directing the flow of enery is base 15.
It's too bad House Bjornaer is such an exclusive case. If more examples existed of non-Faerie beings with more than one natural form, there'd probably be precedent for this, but Bjornaer magi are in a very exclusive boat. Most other creatures with alternate forms use something tangible for the act of transforming, rather than just being multiple things that can be transitioned between. This is probably one of those "up to your troupe" things, unless somebody has something to prove me wrong.
@JL: I don't think the guidelines are intended to be that exclusive. There are rules, albeit extremely vague "here's some examples, do it yourself" ones, for creating spells with original guidelines.
I agree that the guidelines presented aren't necessarily exhaustive. But, in this specific case, I doubt Amy such Rego Corpus or Rego animal effect exists. I'm generally very cautious about deciding on whether such guidelines exist, though.
The spell does the same thing as the Heartbeast ability, and so a Bjornaer has no reason to create/use such a spell.
A Twilight episode will often force a Bjornaer magus into Heartbeast form, can't it? So it's already sort of doable (just not reliably) with a CrVi spell, in one direction, anyway (human to animal).
What about Philipus Niger's (PeVi, Gen) spell The Heathen Witch Reborn in GotF? It says it works on any shapechanging spell or supernatural ability. Does Heartbeast qualify as such?
Neither is ReAn or ReCo, but might be relevant to the discussion.
Often enough published spells need to serve as guidelines. This just saves space in the books. Wizard's Communion in ArM5 p.160 is a famous example. But if there is no guideline or example at all for an effect in the thirty or so books out, this should be really taken as a sign to be cautious and think twice.
This is resolved in HoH:S p.129, where The Heathen Witch Reborn is written out more completely. And no, it "does not affect characters with the Heartbeast ability".
I would think, that ReMe 40 Enslave the Mortal Mind (ArM5 p.152) can induce a Bjornaer in human form to assume the form of her Heartbeast, and an equivalent ReAn (see ArM5 p.120 box for taking over guidelines from ReMe) induce her to change back from Heartbeast to human.
The caster would just needs the knowledge that his target is a Bjornaer, and how her Bjornaer shapechanging works: both of which cannot be had just for casting a standard hermetic spell.
Cheers
Thinking about it, I guess what I'm asking boils down to "Can you, via a spell, actively force someone to use an arcane ability?"
Since Heartbeast IS an ability, with it's own score, putting aside magic resistance, can you force someone to use it, as an ability?
Actually, I'd argue that this is a fundamentally different question. I'd say as long as the duration is long enough, yeah, you can tell the Shapeshifter to turn into an owl and he'll figure out the hard part (assuming turning into an owl is something he can do).
I think the question is more like, "Can you trigger a person's supernatural abilities without their involvement?" Forcing someone to use an ability would be a Mentem effect. What you seem to want is to be able to trigger their ability without their consent.
That too. Are those abilities a part of their essential nature?
Hi,
ReMe will obviously work, to force the Bjornaer to work the change.
ReCo will obviously not work: The Bjornaer in Heartbeast form is no longer Corpus. ReCo(An) might work.
Except that Heartbeast is a Supernatural Ability, right? At the very least Hermetic, but since mundanes can have it, supernatural, sort of the way all those Hermetic virtues become Supernatural when hedgies can take them. So this change can hardly be considered natural!
So Rego is right out.
Anyway,
Ken
Fwiw, triggering a supernatural ability should be a common effect, IMO. Has nothing to do with whether the u dearly on g for of the target is Corpus of Animal.
Also, Hermetic virtues, by and large, might better be classified as gifted virtues of they can bet taken by any Gifted virtue and Hermetic if they apply only to Hermetic magi. I don't consider them to be Superbatural, nor does the RAW.
Unless I am very much missing something,
The original intent was to try to use Rego magic to create a natural change.
Triggering or simulating a non-natural ability is not natural.
As for Hermetic Virtues not being Supernatural, well, sure, they are Hermetic. But except for status-like virtues, whenever a non-Hermetic magus is allowed to take one, the Virtue becomes Supernatural.
Also, I don't see anywhere that ReCo allows the magus to access a target's skills. One can force a target to move, but it's the magus who is moving the target, not the target using his abilities to move. So this notion of using Rego to "trigger an ability" doesn't make sense to me. Far from its being a common effect, I don't notice a guideline outside of using Mentem indirectly that allows it to be an effect at all.
Anyway,
Ken
Sorry. My post suffered from a bit of DYAC, since I was doing it from my phone...
Is instead:
FWIW, triggering a supernatural ability should be a VIM effect, IMO. Has nothing to do with the underlying form of the target is Corpus or Animal.
I'd argue that to research such an effect would require the cooperation of a bjorner magus in creating a weakness in their own house, and therefore is highly unlikely before you even get close to the question of whether it would work.
That's not necessarily true. If elements of the pre-Hermetic tradition Birna left are still extant, then you could potentially have Bjornaer magi trying to research a way to detect/expose them and/or elements of said tradition providing using other houses to come up with a way to help defeat/destroy House Bjornaer.
I don't see how this is any worse than any other transformation spell. They'd do a Heartbeast roll to ignore any ongoing attempts to keep them in one form all the same.
As for who would be willing to share, try anyone ever kicked out of House Bjornaer. I assume there's at least occasionally one who gets away rather than being gutted in their sleep by their former associates.
Hmm. I still disagree with that. You cannot use CrVi to create that supernatural ability, so why should ReVi be able to "trigger" it? While we're at it, let's MuVi it into something else, which would be oh, so convenient: "I MuVi his Second Sight into Shapeshifting!" It isn't as though this ability were just hanging out as its own thing. By this logic, why not use ReVi to remotely "trigger" some other magus' spell... and spend all his vis on it too?
Just yuck and more yuck.
And we haven't even gotten to the other yuck: Shall we have versions of this based on Realm? How about Form? Because, like, Vi works that way.
Did I mention yuck?
Oh, imosvho, of course. 
Anyway,
Ken
Is it possible for magic to force the change? Yes
HoH:MC p16 states that the Gathering of Twelve Years includes a Gothic ritual designed to ensure that no shapeshifters are present because it forces everyone with a heartbeast to assume that form.
I imagine the benefit of this is anyone who is a shapeshifter and not a Bjornaer will not be affected and thus exposed; even trying to time their own shapeshift to match the ritual would probably be difficult.
Can Hermetic magic force the change? Up for debate
My opinion is that ReMe and ReAn could compel a Bjornaer to use their Heartbeast ability, but ReCo might not. It's stated the Hermetic magic cannot tell a heartbeast apart from an animal, not that it can't mentally compel its use.
I don't think Bjornaer would want research into this, it doesn't seem to be of benefit to them.