Currently niggling at an idea that is barely more than idle speculation, so please bare with me as I air my thoughts.
I have some Tom Holt and Wizard of Oz movie mixed up in my head.
What happens if certain non-Hermetic magic users/witches really are another species? Witch covens are organised by troll witches to prepare converts into becoming more troll wives? Say a special ritual to transform the prepared coven members into green skinned, ageless witches, but only during a lunar eclipse when the correct conditions occur, say once every few centuries.
Gruagachan transform into trolls due to Final Twilight. Bjornaer into Great Beasts if they are prepared enough.
Lots of other various attempts by Magi to transform into immortal before riddled with Twilight.
So why not a non-Hermetic route?
Well to begin with Gruagachen are non hermetic.
Beyond that either discuss with your SG or if you are the SG then go ahead and design the tradition- an initiation to gain the virtue transformed being is all it would take. It is even possible that some entity is getting people to transform and build up their might at low levels where it is easy to use them as a snack for the vis...
I had been vaguely thinking that the original trollish witch was promoting witch covens out of some sort of maternal instinct to reproduce her kind .
However, providing fuel to advance her is also a useful motivation
Sounds like a particular brand of Faerie Calling to me.
I just thought of another possible plot reason that a troll-wife/sorceress might groom witch covens to make them undergo a once in a couple of centuries magical transformation ritual into troll-wife/sorceresses (or whatever they are called - jury is out on the name).
Perhaps the original troll-wife/sorceress is from Ironwood, and she is recruiting/producing more (elite) foot-soldiers for the promised Ragnarok against the gods of Valhalla....
This would work best up in the old Norse lands. And I had been wondering about Seithr recently. The idle speculations may be starting to come together.
I should have been more clear. I meant magicians not associated with the Order of Hermes. Aren't most surviving Gruagachan (technically) members of House ex Miscellanea?
No, Gruagachen are a hedge tradition completely not associated with the order. That's part of why their warping results in something other than twilight as the order experiences it.
No, most Gruagachan don't like the Order of Hermes much at all and are not part of it.
A few have sworn the Oath of Hermes and joined the order, but they are the exceptions rather than the rule.
I had a separate idea along a similar line - have a giant who marries a Gifted man (see hedge magic p86 in the Trollsynir section), most of their children will be Trollsynir (giant-blooded Gruagachan) but maybe one of the children is Gifted and rebels to run off with the Muspelli? So you have Gifted dad (maybe hermetic, maybe a hedge mage), Giant mother (using RoP:Magic rules), Trollsynir kids and a rebel Muspelli all as one squabbling family unit. I might do this for a one-shot or a brief saga, I don't know how well this would work longer term.