There is not much personal info on Damhain-Allaidh, but odds on he was a heterosexual male who was not obviously proscribed from sexual activity. Though the Gift penalty might make it difficult to find willing partners.
Still, I am assuming that he left a few descendents.
Having recently read the GM info towards the end of TTT (Thrice Told Tales), I am now wondering if said descendants would qualify for Mythic Blood, or Magical Blood (Magic Spirit)?
Would it be possible to have a character with both blood virtues? Either from the same ancestor, or perhaps two different ancestors - Magical Blood from D, and Mythic Blood from, say, Pralix.
After all, there is no reason I know of that a character cannot have both Faerie Blood and Mythic Blood .
In the 3rd edition book, Lion of the North, there is an actual character write-up for him. Granted it is massively out dated but gives you a good idea of where his rough overview in TTT comes from along with the fact that he was pretty darn scary.
As for any decedents, consider that he has not occupied his own body from the vast majority of his existence. Even if his freaky spider corpse looking self managed to get lucky (or more likely forcefully), any children are descended from the line of the body he took over rather than his own. YSMV but I believe this would greatly limit most cases of Mythic Blood.
Moving on to Magical Blood, that is a "granted" Virtue. By that I mean it is the result of exposure to something within the Magic Realm, rather than a trait passed on by birth. Anyone who can make the trip into the Magic Realm and hang out around something powerful can gain it. I believe what you are actually looking for is the "Magic Human" Virtue. Same book (RoP:M), on page 45.
Magical blood can be either. Yes it can be a person’s minor virtue at Warping 5 because they’ve regularly gained warping from that sort of being but it can also be gained at birth by their parents who have that sort of magical blood whether that is originally the “granted” process gave it to the parent or ancestor or actual mating with a magical human or, less likely, while the animal, spirit, thing is transformed into a human. Similarly to Faerie Blood which can be granted or received due to an ancestor it was granted to or through a variety of mating situations with faeries, RoP:F goes into this in a bit of detail, possibly including a surrogate mother.
"The character’s essential nature has been improved through exposure to the Magic realm, often from a magic being, which gives her true form special supernatural properties."
Nothing in the Virtue even hints at it being something that can be passed on to children.
Also while your group might allow it to be taken as someones minor virtue at Warping 5, that is slightly counter to the description which is about exposure to the Magic Realm. The normal way to acquire it is spending "Vis" saved up in the Magic Realm to represent some change to essential nature from said exposure.
The thing is that once your essential nature is changed, that inherently becomes something that may potentially be passed down to children because it is your 8essential nature* (provided the change does not preclude children)
First off, any new virtue or flaw is a change to essential nature.
Second, I believe that statement is Magic realm in the broader sense of anything magic-aligned, not necessarily visiting the Magic Realm itself. Note that they use realm with a lowercase 'r'. Confirmation of this is on p17 where it says, "(In this book, "Realm" is capitalized to distinguish the place, as opposed to the realm of Magic in general.)" Also, if you notice, its similar Flaw, Monstrous Blood, can definitely be a product of warping.
Third, the Magic Human description talks about giant ancestors.
I would also point you towards the bit about Magic Kin on p89 being lineages of either beings with might or merely magic-aligned people and this seems the perfect virtue to represent those that have this sort of lineage-passed magic alignment.
The virtue lets characters take Magic lore during character creation. Anyone knows of characters with a background in the Magic Realm? I've never seen any. Which hints that maybe, it's not the Magic Realm as a place, but the Magic Realm as a virtue alignment, which can probably be rephrased as "something magical". The Magic Human version explicitly points out to the possibility of this being earned by having the blood of giant ancestors. So yes, it can be transmited through normal reproduction, although it's not the only way (I am challenged by the idea of being descended from a magic thing... absurd Muto non-hermetic effects notwithstanding).
May I point you to Potion of Creation on p56 of Ancient Magic. Sure, an effect attainable with a breakthrough or while trying to create one but it would allow even a thing turned into a man using Hermetic Muto means to help conceive.
It's a nice effect. I don't want to distract the thread too much with the discussion of unrelated mechanics, but as I recall, a man using a Muto Corpus spell to turn into a women isn't fertile, and I don't believe the Potion of Creation bypasses that, let alone for a magic thing.
I kind of thought a Magic Spirit inhabiting a Gifted Human might "grant" the Virtue through simple procreative acts. It apparently takes a decade or more for the Warping to become too noticeable.
I expect such Magical Blood to grow weaker (rapidly?) over the generations. But while the rules of Mendellian inheritance aren't known, I expect genealogists and skilled Animal Husbandists (sp?) probably know that inherited traits can weaken, disappear and suddenly re-appear generations later.
If I recall correctly, one of the reasons non-Gifted characters can have supernatural Virtues is that it is something an ancestor had that they inherit (after skipping generations?)
So for a hypothetical, say the 7th son of a 7th son conceived in a Magic Aura associated strongly with the big D, possibly during a Lunar eclipse and/or other omens (comet in the sky, et al), and all 4 grandparents just happen to be descended from Dav'nalleus, might just spontaneously develop/reawaken Magical Blood.
I don't think I have said/suggested anything that goes against the rules.
And now I feel i should have gotten more sleep, but instead I am looking down this rabbit hole.
It is probably against RAW, but a Bjornaer apprentice with this Magical Blood attends the Ritual of Twelve Years and finds that their Ancestor Spirit is Damhan-Allaidh.
Your Heartbeast is supposed to be an animal and not vermin like a spider....
... says the big member of Clan Wilkis. But there might still be (ArM5 p.91 Bjornaer: The Heartbeast) the Bjornaer maga whose heartbeast is a willow tree.
I have been brought to my attention that while "Damhain-Allaidh" translates as "spider", it is composed of elements that individually mean "Fierce little Stag".
I am of the impression that people of the Mythic Europe era are not necessarily adherents of Mendelian inheritance. The moral state of your soul when you cause the conception of a child is apparently a factor in the child's physical makeup, otherwise why is "bastard" such a terrible curse?
Then there are stories of children inheriting traits from more than one father - Castor and Pollux from Greek myth, and how the Quinotaur is involved in the origin story of Merovech.
Hence I am of the opinion that any body while inhabited by D's spirit will produce D blood in children sired. Whether it be Mythic Blood or Magical Blood.
After all, the body starts accruing D's warping points once D takes up possession.
Though I make so many mistakes about the ME mindset, I welcome correction.