Let´s assume one of your players wants to play a Tytalus with a concept similar to the "Faust" by Goethe (plaything between the forces of good and evil, tries to be at least a little bit more than a tool and to be able to take an active role into this game). I think, for a Tytalus this is the highest level: the conflict between good and evil... it´s the most fundamental conflict I can imagine. When the Magus enters the game, he knows, that he is adviced (and sometimes directed) by a supernatural entity (~ plagued by supernatural entity Flaw), but he isn´t sure about the nature of the entity, that speaks to him. Furthmore it is set, that the Tytalus killed his parens at the end of his apprenticeship (as it is not too unusual in house Tytalus).
Do you have some ideas for creating a character like that? At present I talk with the player of the Tytalus about the creation of this character and could do with some ideas from othe players.
The Titanoi (HoH:S p.94f) and Tasgilia (p.74) appear to modeled after literary Doctor Faustus. Being advised by a supernatural entity that might be infernal is still likely to get a magus marched, so you need to be very careful to construct his background.
If the PC magus' parens was a 'diabolist', the PC magus might have somehow killed him to not become his prey - thereby emulating Tytalus and gaining esteem and compassion in his House. If the Quaesitores and his Tribunal let the PC magus live after that, they share now some of his burden, but he still has the Flaws Infamous Master, Diabolic Past and/or Favors (Quaesitores). Later he might have become the target of the machinations of the entity 'advising' the parens, and perhaps an antagonizing entity. Best prepare your player for a lot of in-character angst in time - as his sodales are still forever suspicious.
It is entirely possible and reasonable to have a past with a master as described and not have to take either Diabolic Past or Favors. Story Flaws are flaws that the player wants to play, and not compulsory in any manner. Infamous Master is more than sufficient to cover such a character, perhaps even with the addition of Infamous which means that he's cursed generally by others (his family).
This can actually fail pretty easily, since the Primus Tytalus (or any other Tytalus, for that matter) can just declare him a member of House Tytalus and sponsor his membership within the House. Obviously slaying one's master is a feat worthy of a Tytalus apprentice becoming a magus. So, while house Tremere might do something to claim him, there's nothing to prevent House Tytalus doing the same. He can then easily renounce his membership of House Tremere, should he so choose.
That's what the "and/or" stands for. Chiarina will sort that out with the player: no need for us to provide more than suggestions.
Remember, that the Primus of House Tremere holds Noctivagus' voting sigil?
It is usually not that easy to leave House Tremere. You would have to come up with a Tribunal accepting to replace that sigil for your scheme, thereby creating a magus with two voting sigils at the same time, who thereby violates his Oath "I will have one vote at Tribunal, and I will use it prudently". So you would run a rather particular campaign, wouldn't you?
Let's say, the Primus of House Tremere knows that this is not going to happen anytime soon - and certainly not in a Stonehenge Tribunal dominated by Blackthorn.
Anyway, that type of campaign specialties doesn't concern chiarina's questions.
Or presumes at least one of them in the list, which isn't necessary. Let's provide suggestions, then, not list requirements. Your list seems to be more a punchlist than a suggestion list.
Uh, what? That is a possibility, not a certainty. Let's work again, in suggestions, rather than requirements.
It is equally possible that the Primus is requested by the friendly neighborhood Quaesitores, since the person is no longer a member of the House, and thus no longer legally bound by the rules of the House. My sense of Tremere is that they wouldn't even need the Quaesitores to ask, and would send it with a commentary of Good Riddance to Bad Rubbish note. And then they would renounce his master and expel him from the house, posthumously.
The "rules of the House" should not concern the Quaesitores. But a magus with two voting sigils will concern the Tribunal: so he just has to get that sigil he voluntarily rendered back first.
Obsecrator the diabolist was certainly condemned by all the Tremere, long before the Quaesitorial proceedings of his crimes were finished.
But Tancred in the hands of House Tytalus, as a magus or not, is at the very least an embarassment to House Tremere: he cannot yet put his knowledge into context - but as he slowly begins to understand and play the game of House Tytalus, his recollections might be used to rebuff the allusions to Tasgilia some magi of House Tremere are fond of. That made the Primus of Tremere act. And that makes the Tremere - Tytalus conflict interesting, if one wishes to tell complex stories of a diabolist's apprentice. If Tancred thinks higher of House Tytalus than House Tremere when the investigation of Obsecrator is concluded, there are ways for him to join House Tytalus instead.
No need to worry. I´m aware that it aren´t the ten commandments One Shot is writing.
The thing about Noctivagus I have to consider is the sequence he is acquiring his Virtues and Flaws. My player told me, he´d like to start with "plagued by supernatural entity". One Shots suggestion was to start with a virtue, that says something about the apprenticeship of the magus (Infamous Master, Diabolic Past and/or Favors...). There are good reasons that the magus would acquire "plagued by supernatural entity" later. Now I have to think about, how far the Magus has just travelled on his road to the pact with the devil.
The concerns of Tremere are not the concerns of Tytalus. Tytalus would love nothing more to disrupt Tremere plans, and adopting such a character into the house is eminently reasonable, even predictable.
I'm not claiming the magus would have two sigils, you are. There are two paths here, one would negate the Tremere part of Noctivagus entirely. He's adopted into House Tytalus and gauntleted as a Tytalus because he killed his master. The other path, is to also be declared Tytalus after making a deal with the Primus Tremere. If, after joining House Tytalus, he were to renounce membership in Tremere, the Primus really doesn't have any rights to retain the sigil. Certainly the Code doesn't provide him rights to keep the sigil, and it's a custom of the House, more than anything. A Tytalus, having renounced membership in House Tremere, is entitled to his sigil and may even seek the help of Quaesitores in the process. That's it. There aren't two sigils, ever. HoH:Societates, even details magi of Tytalus declaring others to be members of the House, much to their chagrin. These people do not have two sigils, nor is it even suggested. Your logic here is really flawed.
Plagued by a Supernatural Entity is a challenging one for a magus who stays within the covenant, if the entity can't get into the Aegis. Of course, whenever he leaves the covenant, for any reason, it's a big deal and a big opportunity for plaguing...
Not so. Once a filius renders his sigil to his parens under the condition, that he only gets it back once he defeats that parens in certamen, this condition is in no way affected by the filius changing House.
Of course the Transsylvanian Tribunal, where the Primus of Tremere resides, can in special cases decide differently.
Noctivagus is an ArM4 magus (clearly flagged already in https://forum.atlas-games.com/t/faust-as-a-tytalus-magus/10077/1). So you nitpick quoting the wrong ArM version here. For the power of the Primus of Tremere, and the possibility to leave House Tremere, best read ArM4 The Wizard's Grimoire p.48. So this part of the discussion is quite superfluous and does not concern the OP at all.
A filius is a magus, no longer an apprentice.
A sigil becomes valid immediately after the gauntlet, as the sign of being a magus able to vote. A Tremere parens keeps it or delivers it to the superior in the House holding the parens' sigil. (See ArM4 Houses of Hermes p.103).
One Shot, you can argue, or nitpick, that as a matter of ArM4 canon, you are correct, or maybe. I don't have Wizard's Grimoire, as it's 4th edition product, and generally superfluous to my needs in 5th edition. It may say that House Tremere can retain the sigils of magi who retire from the House without claiming their sigil. I don't know. I don't care.
It is certainly reasonable to argue that in a game of 5th edition, which I think we can reasonably presume that most folks are playing and working with, that what you outline is easily defeated in 5th edition canon. Qualify your arguments all you want, that you are arguing for 4th edition. In which case, you aren't really discussing a current game system, and are arguing in your own little world.
I don't have to read better, and more precisely, One Shot.
I can point out the flaws in what you suggest in a current canon, because I presume, unless it is clearly labeled that all threads are in the current canon. Note, I'm not saying you weren't arguing it was 4th edition canon. It matters not, because it was the OP that opened the question, and it is reasonable to presume that he is asking within the Ars Magica 5th edition context, not 4th edition.
Edit: I'm posting from my phone, so I fat fingered One Shot's name. I noticed he was logged in before I edited the post with a correction.
The example was labeled reasonably early and clearly.
And you didn't even address chiarina's concerns. You just overlooked an explicit label, which "it is reasonable to presume" that a reader notices. And then you discussed Tremere orbi and sigils in the wrong version, which in any version would not have concerned chiarina at all.