SOCIETATES tells me that the current Primus of Ex Miscellenia is a descendant of an Orbus or Orphan that is to say a Mage from another House who left that House for some reason, perhaps political, perhaps philosophical.
A political reason might be a Tremere who did not agree with the decision to expel the Diedne or a Guernicus who did not agree with what that house did to bring the Schism War to an end.
A philosophical reason might be a Criamon who has decided Empedocles was full of it.
Is there a canon answer to what sort of Orphan Primus Ebroin is descended from?
If there isn't what might be an entertaining answer?
The schism war is a bit ancient history for a current primus to be Orbus over- most were born after the schism war.
There is not a canon answer as to which house they are from, it is left as a plot point for the SG to decide, along with whether or not they are truly orbus versus an infiltrator...
The current Primus is the descendant of a line of magi who joined Ex Misc from another House after some disagreement deep in the past.
Given that House Bonisagus is still following the division of labour set up at its founding by Bonisagus and Trianoma, I think history tends to be a matter that the long-lived magi keep with them for a long time.
And it would decide the specialisations and secret knowledge the Primus has... And which he is keeping close to his chest.
that assumes the primus is raising an apprentice (or raising one as an ex miscellanea if the personae model is used)
the main story based issues for the primus being orbus are if they are 1) Tytals, and being orbus is a tactic or b) Tremere with the same issue, but with more of a false flag concept.
Is this intended as a hypothetical or canon? If the latter, my read is that it is him who joined Ex-Misc from another House rather than a Hermetic ancestor - HoH:S pg 103 states:
Ebroin is an orbus (see below), although he prefers it that this is not widely known. Furthermore, the circumstances of his expulsion from his House are not altogether clear, and there is a hint of a suggestion that he may still be loyal to his former Primus...It is left to the storyguide to decide upon Ebroin’s former House and his future plans."
"These magi (or their ancestors) began their careers in a different House."
Now what it says on p 103 does seem to indicate it was Ebroin who skeedadled but you can see why I was thinking otherwise. A magus who both manages to flee his original House and get appointed Primus before he is thirty is clearly a remarkable fellow.
Still assuming it does mean that he's a first generation Orbus what would be entertaining?
Tytalus, yes. Never quite sure if they've really changed their stripes and the same can be said for Tremere. I think in both cases it's more fun if they really, really loath their former alleigiance (there's a lot to dislike in both cases) but can't get anyone who knows to trust them.
Bonisagus: perhaps they found the supervision of their research intolerable? Perhaps they have a particularly dangerous breakthrough? Or their teacher gave them the results that were never reported to the House hierarchy.
Guennicus... Maybe. Not many obvious points of rebellion that would drive a lawyer to leave. The factions haven't yet resolved their difficulties there's all to play for.
A Criamon who is tired of the mystical nonsense? But who nonetheless knows some of their stranger secrets. The tatoos are a bit of a problem
Not Bjornaer nor Merinita: it would be too obvious. Likewise Veriditus.
Flambeau? A Flambeau peacemaker?
Jerbition? Who could possibly object to being part of that civilised, aristocratic and well financed house?
Mercere? Not too likely given the way they prize their few Gifted members.
If you want to go for the classic, he is of course actually Diedne. After that the cover House becomes less important (although is probably more likely to be Bjornaer or Merinita - note you don't have to have Faerie Blood to be a Merinita (and minor Faerie Blood isn't necessarily obvious), it's just common.)
Some of the Ex-Misc factions were pretty anti-Diedne. Some of them may have been more pro (although honestly, I'm struggling to remember anyone who is canonically down as actively liking the Diedne, as opposed to just being worried they'll be next, disliking their enemies or thinking that killing them all is a bit much - maybe the Bonisagus lineage Bonisagi?). Time to see what he can do about rebalancing the House into more favourable weightings...
Guernicus and Bonisagus both believed that what happened to Diedne was a great travesty, Bjornaer and Ex Miscelania were both worried that the "Latin club" would be coming for them next. Tytalus supported Diedne because the Tremere were the ones leading the charge against them.
Ebroin has all the hallmarks of a classic Tremere plot. House Ex Misc was in terrible shape before Ebroin took over. If the house were to collapse, that would be bad for the Order. So, instead, House Tremere has a plan to plant a Primus of their own, to save Ex Misc from itself. One of the first things Ebroin did was order a census of the house, to find out exactly how many magi there are and where they’re located; this kind of organizational emphasis is standard operation procedure for Tremere. The thing to remember about the revised Tremere is, they do the right thing for the wrong reasons. Ex Misc is rebuilding, it has new energy with Ebroin at its head. Good thing. But done for the wrong reason.
Of course, then there’s all the politics and intrigue surrounding Ebroin and an inner council he can’t control. I suspect he might not have been the original choice for House Tremere; he may have gotten assigned this job as a fluke, or by a superior with influence in the house, before he was actually ready for it. Which makes a better story anyway.
Or it could be a Tremere plot to use the Ex Miscelania house to create a situation that will lead the grand tribunal towards ruling in a way the Tremere want, such as to standardize all tribunals to Latin as an official language. Or perhaps he was sent in to search for any trace of Diedne magic amongst the ExMiscelania, or to investigate rumors of another tradition practicing human sacrifice...
Oh for sure, House Tremere would have plans for Ex Misc. It is canonically the largest House, and the census would only increase that size. This would allow House Tremere to influence Hibernia and Loch Leglean, two tribunals they otherwise have almost no presence in. Plus, as you note, the obvious benefit of two Primus votes at Grand Tribunal. Ex Misc are notoriously unlikely to follow the lead of their Primus, and I see no reason why that would change, so it’s not like Tremere can count on hundreds of Ex Misc votes. But it’s a start!
The title of the HoH:S p.103 Saga Seed is The Boy King. Hence it makes sense to look up the historical English 'Boy King', Edward VI, and his rule.
The 16th century 'Boy King' was an ardent protestant, and over his and his council's rule institutionalized protestantism was firmly established in England:
The Ordinal of 1550 replaced the divine ordination of priests with a government-run appointment system, authorising ministers to preach the gospel and administer the sacraments rather than, as before, "to offer sacrifice and celebrate mass both for the living and the dead".[131
Such a reform from the top would be very much Tremere style: not in religion, but in magic and Hermetic politics, of course.
If Ebroin is a Tremere plant, Goliard of Tremere (Heirs to Merlin p.120 box) is likely to be an intended beneficiary of his tenure as Primus ex Miscellanea, using it to firmly restore Blackthorn in the Stonehenge Tribunal.