Is there an official reference in any of the rules as to what happens when a Tremere magus takes the Mythic Blood virtue?
I have been away for a while, and I am not certain which book it is in, and my Search-Fu of these fora hasn't provided an answer yet.
To re-iterate the problem, Tremere magi get a free minor magical focus in Certamon, and the Mythic Blood virtue also gives a free minor magical focus in something. Yet the 5th Ed rules say a character can not start with more than one magical focus.
The 'official' rule (ArM5 p.46, spelled out twice) indeed is simply, that the result is not possible: accumulated Foci constitute a balance issue.
The Tremere 'weakness' of the Minor Focus (Certamen) is built into the House (HoH:TL p.131 Virtues and Flaws). So in practice you need to ask your troupe, before you propose that exceptional Tremere magus with Mythic Blood, and eventually run a house-ruled character.
TMRE allows Initiations into new Magical Foci. It proposes to teach a maga, who already has another Magical Focus, the Potent Magic (TMRE p.31f) equivalent to the new Magical Focus instead. You could follow this example for your exceptional Tremere as well. While Potent Magic (Certamen) does not make sense, Mythic Blood leading to Potent Magic does.
As house rule, we have decided the "focus in certanem" was not a real focus but a high capacity giving the same effect and so the player could have a real focus for mythic blood or else
You also have the "potenty magic" for your mythic blood instead of taking minor magical focus, you could take minor potent magic which gives a bonus of +3 instead of doubling the smallest art in the speciality.
I had been hoping that there was a better answer than that, but thems the breaks. Kind of sucks that Tremere characters are denied a couple of Magic Hermetic virtues.
I will try creating my Tremere character with Flawless Magic instead of Mythic Blood.
BTW I am giving him the Special Circumstances - when the recipient of unfriendly magic. I was originally thinking during Wizard War (I am considering modelling my character after the Red Baron, or other stereotypical German military officer), sort of like he does his best work while under stress, but might this also apply to Cetamen duels?
Yeah, the Tremere section in the HoH books (or in Against the Dark? I forget) mention this - that their house virtue kinda sucks if you don't do a lot of certamen; and that if your Saga doesn't use it, then it should be made up in terms of access to house resources. In contrast, if there is a lot of Certamen going on, then the house resources should be proportionally restricted.