Arhain is the Eltharin word for Shadows/Stealth/Perfidousness etc. It clearly refers to the Forest of Shadows, except it's in Eltharin. The first thought is of course, the patron goddess of the Forest of Shadows, Haletha. The intriguing question however, is why the inserted thought was in Eltharin. Is it perhaps true that Ranald/Shallya are also Elven Gods and that poured over into their child if Haletha is truly the child here? Or is it some Elven God/dess that isn't Haletha that's also in this forest? There was a Cult of Styriss in Ostermark relatively recently that worshipped Anath Raema, but I somehow doubt Anath Raema would be fighting back against Drycha.
I'm not sure that we should make much of the thought being in Eltharin. It might be that Halétha - or whatever force is at work here - is implenting the abstract idea that the Forest of Shadows is opposed to Athel Loren, and the exact shape that this takes in the recipient's mind depends on them. Maybe Mathilde just thinks in Eltharin when thinking of magical forests? Her frame of reference for those things is Laurelorn, after all, and the other forest foremost in her mind at the moment is obviously Athel Loren. And we have numerous instances of Mathilde using plant based metaphors since she began spending time in Laurelorn, sometimes even when speaking Reikspiel, which I think indicates Mathilde's inner dialogue is in Eltharin at least some of the time.
If we are to ascribe meaning to the Eltharin then one possibility is that the forest itself is speaking to Mathilde. Perhaps the Forest of Shadows communicates in Eltharin when it communicates with mortals at all, because in ancient times it was always elves that spoke with the forest spirits. But from what I can gather the Forest of Shadows seems pretty malevolent, and while it may be hindering Drycha I don't think it would outright assist Mathilde, who is also an Ulgu user not of the forest and could be considered an invader herself (though unlike Drycha she isn't attempting to subvert the spirits of the forest to her will so maybe the forest is being pragmatic and working with the lesser evil?). I think Halétha, Goddess of Protection from the Forest of Shadows, is more likely to whisper advice into the mind of someone opposing destructive forces active in the forest.
On the other hand, it might be that Mathilde has unwittingly channeling the spirits of the Forest of Shadows, just as she did in the Mork heist. No accidental ritual this time, but she's an Ulgu user with an arcane mark that makes shadows cling to her and she's acting against the invaders, and with her avatar trait that might be enough resonance to 'hear' the spirits of the forest without any intentional decision on anyone's part.
Anyway, regarding the vote:
[X] Spellcasters
I haven't caught up on all the discussion but I think that I would prefer to face the leader of the opposing force with an army on our sides. Besdies, riding into the forest in search of the leader will take time, and in that time the spellcasters might act unopposed and seriously harm our forces. If Mathilde quickly dispatches the spellcasters we can reassess, and if she doesn't then it means they aren't that easy to deal with in which case we'll be glad that we didn't leave them alone.