Hello everyone!
I'm trying to build a blood magus and I was thinking about taking a Focus for blood related spells, do you think it's minor or major?
There is an interesting virtue too called Exotic Casting that I think it could work well if paired with a power like purity or debauchery that makes you cast by hurting yourself. (Sword made out of my own blood? Can it get more metal thank this?)
Any advice or idea is more than welcome! Thank you.
What is 'Blood Magus' to you? Just a magus themed around blood? Sacrifices? Other?
Mostly a magus related to blood magic.
I was thinking about using his own blood to make sword and armor, manipulate the enemies blood making it boil or flow out of their bodies and using it to empower himself.
Sacrifices are not out of the picture but I don't really like the way the infernal manual depicts sin and the way you become distorted. (All the interpretation of Infernal magic come from a Abrahamic's religions perspective which is very flawed and based on wrong translations of the Bible and wrong interpretations of God, good and evil which I don't really like)
This sounds a lot like you want to make a magus who focuses on spells that affect blood.
In that case a magical focus in blood is a very good idea.
You will also want to specialize in corpus magic as blood is most certainly a part of Corpus, and Animal if you target animal blood.
You might want to dip into some of the more esoteric mystery cult things, but I would keep that to a later point in the game for various reasons.
Do you know if a Blood Focus is minor or major? Rules aren't clear at all and some of the exhamples given trample said rules.
You will have to consult the guidelines for the respective virtues in the core rulebook. However my gut feeling is that it is a major magical focus, as its application spans one form (Corpus) but all Techniques within that form, i.e. it applies to all of Creo, Intellego, Muto, Perdo and Rego Corpus.
If you decide to include animal blood, I would allow that as a Storyguide, even though it would stretch your focus to apply to the form of Animal too.
I could see a case for minor too. Especially given that Self-transformation is a minor magical focus, and IMO Self-transformation is much more useful than blood. However as you can probably guess there is a pretty wide grey area and you can only resolve said grey area by discussion with your troupe.
To me Focus in Blood feels minor, because of how limited its application is amidst corpus. Despite the mystical connotations of blood being a source of power, the focus in blood would not aid any standard spell from the book. I feel the 'one form all techniques' metric isn't particularly useful in most cases, since 'Wolves' as a focus is minor, 'Birds' is a major, and 'My beagle Piddles who I sacrificed in a mystery initiation' is also probably a minor.
Thank you for the help, do you guys know any synergistic virtue with the concept?
The character will hurt and fatigue himself s lot
I would second Euphemism's assessment that Blood should be about right for a Major Focus.
Remember, a Major Focus should be less than half a Form; a Minor Focus less than a TeFo combination. Note in this sense that "self-transformation" is, roughly, those MuCo and MuMe effects with R:Self, plus a very few other effects (e.g. CrCo/Me characteristic improvements, and the occasional PeCo/Me effect to remove "unwanted" aspects of oneself, such as weight or a memory). So, it's fair to say that it's less than a TeFo combination.
Blood involves Corpus and Animal -- pretty much all Techniques. It can also cover a handful of applications of other Forms (e.g. the scent of blood is Imaginem, and a ReVi spell to concentrate the vis in a dragon's blood into a single droplet of it is also covered), but let's ignore that. I'd eyeball human blood at less than a quarter of Corpus (so it's small enough for a Major Focus, a bit less than a quarter Corpus and a quarter Animal), but not less than a tenth: many Perdo effects can be worked with blood; a significant fraction of Intellego ones; a few Muto, Creo, and Rego ones.
You might want to look into Leper magi (tytalus), out of the order of hermes societies book either for ideas or the good self hurt
What about "life-linked spontaneous magic" and "life boost"? Those fit nicely into the trope of blood magic. I.e. you can boost your magic by taking (fatigue) damage.
This is true but also kind of not true. I would argue that many if not most of the PeCo spells in the core book could easily be reinvented to benefit from a focus in blood. Likewise for a significant portion of the corpus spells in other techniques.
@ezzelino: but not less than a tenth: many Perdo effects can be worked with blood; a significant fraction of Intellego ones; a few Muto, Creo, and Rego ones.
There really are quite a few spells to invent. I would like to add a very noteworthy spell I would invent if I had a focus in blood: Bloodbending. Essentially "strings of the unwilling marionette" but through control of the blood.
In general I think a focus in blood opens up a significant number of spells , but mostly a few really powerful spells that the character can quickly learn to cast reliably and with a decent penetration. If it was me I would spend seasons mastering those spells.
A focus in blood is also very useful in finding and taking arcane connections.
This was my first thought as well.
Blood being a source of sympathetic connection I'd say a focus in blood is indirectly useful to any spell casting.
There's possibly a problem with targetting there - I think you'd need Target: Part (of the body) unless you were able to perceive the person's blood directly. On the plus side, an InCo spell to see someone's blood vessels would benefit from the focus...
A Personal Vis source that was the character's own blood (possibly extracted via some sort of ritualised method) might fit as a virtue. There's also various ways to build a Necessary Condition involving blood.
There is going to be a lot dealing with target: Part in this focus. But I think that the bonus provided by the focus will more than offset the added cost of having to use target: Part in most Corpus spells.
A personal vis source of blood is also a great idea.
Thank you all for the beautiful insights 
You gave me a lot of inspiration