If a Bjornaer in swan heartbeast form comes under the influence of a day duration MuAn spell that changes her into an eagle, she assumes the form of an eagle.
But what happens if she tries to assume human form again before the spell ends?
Does she
a) become a human (and the spell goes on, so when she assumes heartbeast form again she will assume eagle form)
b) become a human (and the spell ends, so when she assumes heartbeast form again, she will be a swan)
c) remain a swan (in eagle form because of the spell)
d) become a human (in eagle form because of the spell)
Specifically, changing from Animal to Human or Human to Animal undoes all of the physical transformation effects on the person, with a few notable exceptions, such as spells which rearrange the balance of the Humours (Form of [Temperment] Heartbeast) or spells which directly effect the mind, as the line between human and animal mind for a Bjornaer is blurry.
The correct answer is (B) - she will become a human and when she transforms again, will be a swan.
Would the spell not go into abayence rather then lapse.
For example a PeTe with sun duration would create a whole and prevent it being filled rather then keep on creatng holes. Is it not the same that the target would not be valid and stop and if they resumed an animal form, the spell would kick back in.
Actually, HoH-MC states, p.22, under 'Ringing the Changes': "Animal spells cast on the heartbeast remain in effect after transformation to the human form". However, as Ravenscroft noted, the act of changing form between the heartbeast and the human while under a shapechanging spell is special, as described under Changing Form on the same page: it takes a roll, but if successful the shapechanging spell is canceled.
Actually, if you read the Bjornaer: the Heartbeast section of the core rules, p.91-92 it says the same thing: "A Bjornaer that has been physically changed by magic may try to assume her normal form or her heartbeast form. She must roll [...]. If she does resume one of her natural form, the other magic is dispelled."
So a shapechanging MuAn spell makes it more difficult for a Bjornaer to go back to his human form, but if he does succeed the spell is dispelled. If, however, he had been hit with a ReAn spell, say some kind of mind control, it would still be active and in effect after the transformation, despite the fact that the Bjornaer's mind had technically changed Form during the transformation.
"Animal spells cast on the heartbeast remain in effect after transformation to the human form"
If she does resume one of her natural form, the other magic is dispelled.
The second one specifically talks about shapechanging spells while the first one doesnt, but you could easily make them clash anyway.
In my mind, core rules beats additions...
I recommend you read the actual text of both, I only quoted part of it. What it specifically says is that you can use your Heartbeast ability to dispel a shapechanging spell and revert back to one of your natural shapes with a specific roll tied to the magnitude of the shapechanging effect you are under. This is not just about getting back to your normal shape, this is about using your Heartbeast ability to dispel a shapechange. In fact, your Swan Bjornaer changed into an eagle could try to resume her swan shape, and if she succeeds she will be back to a swan with the eagle spell dispelled despite the fact that she is still an animal as far as magic is concerned. Remember, there is normally no roll for a Bjornaer changing between his heartbeast and human forms: the roll in this case is specifically to beat the shapechanging magic.
Say the target therefore had the ability to become an animal such as skin or shape changers. Would a MuAn spell or MuCo spell have the same effect on them. For example would the spell go into abeyance whilst the form was shifted.
Only the heartbeast let you switch from Co to An and back. A shape changer is either an animal in human form (both targeted by An) or a human i animal form (both targeted by Co).
If you cast a MuAn spell on a human shapeshifted into an animal who then returns to their human form while the spell is still in effect (if they even can), then I'd say the spell would not just go into abeyance but be cancelled entirely as it no longer has a valid target.
If you cast a MuCo spell on a human shapeshifted into an animal then the spell will run its course regardless - since the target is always valid for the spell.
Vice versa for animals shapeshifted into humans.
Not RAW, just my reading of the situation.
By the same token it should also hold true that a spirit that has adopted a non-spiritual form via magic is still targetable by its original form (mentem for ghosts, vim for demons, etc.).
Skin changers and Shapeshifters remain humans while in animal form, so Co and Me can still affect them, and any such spell active before the change of form remains active while in animal form.
I don't recall reading any specific rules about what happens to Animal spells cast on them while in Animal form when they return to human form, however I too would rule that they would end the moment they change back to human form.
This is true for Lycanthropes as well, except that the mind of a lycanthrope is Animal while under the change, so Mentem effect do expire.
Active Corpus and Mentem spells on a person who changes via spell or item into an animal remain active.
Spells active on a Bjornaer remain active when changing in any direction.
A Bjornaer can use Heartbeast to cancel a shapechanging effect on them.
#3 is clearly distinct. Let's look at #1 and #2. It seems most consistent that a spell only needs its target to be valid for its Target when cast but then runs its course regardless. This allows #1 and #2 perfectly well, including the invalid later Target that #2 creates. It extends what's above to other shapechangers and to shapechanging using other Forms. It's also a very simple, general rule that can be used to handle other similar situations. Finally, it prevents many effective immunities that would otherwise be provided by just a few low-level Momentary Muto spells. As such I feel this is the intent.
So by that, if i use a ReTe to affect a big piece of stone, someone else turns the stone into fire, then i keep affecting the fire as if nothing happened... Sorry, that just goes weird.
First, remember that the rock in your example is both a rock and a fire, so that makes perfect sense and is very, very strongly implied (It's explicit for certain combinations, but the books didn't list all 90 combinations explicitly.) by the rules. You could even cast your ReTe spell after the stone was turned into fire and you would be fine.
Second, if you ReCo a human Bjornaer and the Bjornaer becomes an animal you still have your Rego effect on the animal, even though the animal is only of Form Animal and not at all of Form Corpus. That is explicitly true by RAW. And here we don't even have the dual nature as above.
Note, I would consider certain effects effectively suppressed. For example, a spell to make your tail prehensile cast on you when you're a cat would remain while you are human but would not do anything because you have no tail. But if you were then to transform into a monkey the spell would become useful again.
