So, I have a player that is quietly behind the other player's backs descending into ebil dark magics.
He wants to make some kind of a breakthrough to incorporate killing animals and people to power magic. Now in Mage the Ascension, it's really clear that all living beings have quintessence (vis) that is part of their life force. People/animals can be killed to power magic.
Not so much in Ars Magica. However, I want to humor my player and allow him to discover way of harnessing the power of killing someone into magic.
My initial thought is a Major Breakthrough - that killing people can power a ritual spell without vis, or perhaps killing enough people allows a ritual spell to be cast as a formulaic spell.
Other then that I'm not sure what else. The player himself just "wants something cool". He also plans on going evil enough later on that when he's found out the other PCs will probably Wizard's March him.
Make it ridiculously easy. Because it the vis he obtains is actually from the infernal, not really from the "life force" of the animals and humans he sacrifices.
This wouldn't work for animals, but you could do something akin to this with, I'd be willing to say, a Minor Breakthrough (or maybe two) and some creativity. Bind their spirit into their body, and ritualistically (not Ritually) torture them to and beyond death, causing them to either become a ghost bound by their desire to be free of the pain or a vendetta for revenge against the magus, or at the very least spawn a Mentem spirit of agony. Either way, you now have a being with Might, and can sacrifice it with the vis inside it to power your Ritual. Several people can be used for particularly large Rituals.
My major thought is that in my experience these kinds of scenarios descend into inter-player bickering and are just not fun. But if it will be fun for you lot - great.
More on point - The Mysteries Revised Edition offers the Mystery of Hermetic Sacrifice, which can substitute for between 1 and 6 pawns of raw vis for powering Ritual spells.
Realms of Power: The Infernal suggests a sacrifice can add +2 to +10 to the casting total. It also offers Cthonic Magic, which allows the character to add 12 x Ceremony x (Characteristic + Method) to his totals by establishing an Infernal coven, which is always nice, and to enjoy the benefits of the high Infernal aura he can (surely!) establish. If you count a proper sacrifice as a "sinful act" for these purposes, then Cthonic Magic can allow him to use sacrifice to double his lowest Art for both Lab and Casting Totals. Really, I'd just have him be initiated into Cthonic Magic and tempt him with grimoires of demonic True Names, Infernal artifacts, a way to erect a secret powerful Infernal aura in his lab...
Two references spring to mind-
fertility magic- the fertility token is infused with 1 pawn of creo vis, but if it is harnessed the woman loses her ability to bear children
ROP:infernal- a sacred object can be corrupted and harnessed for vis with the application of confidence points
life is sacred
also confidence points may be gained through a season of recreational sinning.
OTOH- a creature with might 5 has 1 pawn of vis (typically)
animals with no might are treated as might:1+confidence score
so my suggestion would be that sacrifices be treated as 1+confidence+faith points might, and every time you reach 5 might you get 1 infernal vis from corrupting that much sacred life force.
I find it a large stretch to call life a "sacred object."
Sacrificing people can get some nice bonuses, especially with Chthonic Magic where it's almost broken. But vis is vis. And people just don't have vis. You could do as Amazons do and skip harvesting the vis, electing to sacrifice Magic (or Infernal) beings, but at the end of the day, vis is concentrated Supernatural power. Mundanes by their definition don't have that to contribute to a Ritual.
Well, if a Leper Magus dies for their magic, they get 15 vis out of it. Of course, as far as I can tell, this is at least partly Divine in nature (as it's based in leprosy).
The easy way to do this is Chthonic Magic. If you want to allow death to release vis in the casting of a ritual, that sounds like a custom Mystery Virtue that allows killing someone as part of a Ritual spell to provide vis for the casting.
One obvious set of places for him to start looking are Diedne sites - after all, they (may have) practiced human sacrifice in their rituals. Of course, he'll have to be careful to avoid attracting quaesitorial attention...
As a note - the group does know this PC is up to something, and it's likely not good. But we're being OPEN that there is a big secret going on, so it will hopefully end in good fun.
Sub Rosa #13 has a Diedne tradition that gains vis from sacrificing magical animals.
It says, "You can sacrifice a Magic or Faerie creature as a part of ritual spellcasting. Doing so destroys the creature and renders 50% more vis (of the type embodied in the creature) than would be ordinarily gathered from the creature after its death. This adds an additional botch die to the casting for each pawn of vis generated this way."
Speaking of Diedne, you might also want to look at the Bonebiting Certamen technique in the Tremere section of HoH:TL for some ideas about how blood magic was at least partly integrated into Hermetic magic.
I made two Ex Misc Trads with this: one with the Painfull Magic Flaw (to cut yourself) and Life boost (can bleed more toe boost their formulaic spells), and the Healing ability like Major no-Hermetic Virtue, this are Blood magicians. The others are Bloody mages, more related to the dark sorcerors from Dragon Age video games or L5R maho tsukai; or you can call them Cruori magi: they take the Necesssary Condition (spilled blood), the Special Circunstances (spilled blood) and between Decauchery or Chtonic Magic.
When people first think of "blood sacrifice" they tend to head towards "ritual murder" as a first base. Not that I've any particular interest, you understand, but I've thought about this a little. There's a scale involved and I think the Bonebiting reference is evidence of this.
There are Light, Medium, and Heavy Wounds available before we even get to Incapacitated. Inflicting those on self or other could potentially give different results in terms of generating if not raw vis (I'm not necessarily a fan unless there's an outside power rewarding the act in some way) then an increase to a casting total. Then there's inflicting some kind of flaw; lame, missing eye, blind, etc. You have Minor and Major to play with there. And then of course there's some kind of nebulous "really unpleasant thing you subject a blood relative to for the sake of powering your magic".
Lots of different options before we hit outright murder.
I wonder if it would not be possible to "reskin" Spell Binding into a component of Blood magic.
Instead of binding spirit or elemental to sustain a spell, the magus makes a blood sacrifice of an animal/human being/creature. He would need to design the equivalent of Sustain a spell of (Form), for example "Bind by Blood and (Form)". The size or the quality of the sacrifice will determine the level of the spell which can be sustained.
Another option is to create a two news virtues that can be taken with either Life Boost or Life-Linked Spontaneous Magic called "Blood sacrifice" and "Reserve of Blood".
"Blood sacrifice" allows you to use the benefit of Life Boost and LLSM withtout draining fatigue from you but from a sacrifice. The sacrifice must be either willing or restrained and the fatigue loss are instead wounds level. In the case of Life Boost, if more than one level is drawn, the sacrifice must also soak the damage. If it takes more wounds than it has left, the spell fails (or maybe even back-fire ?).
"Reserve of Blood", the magus can store magical energy through sacrifice to power up LB and LLSM. During a ritual sacrifice (again from a willing or restrained victim), the magus inflict a wound of his choice (Test of Dex + Chirurgy/Philosophae or other appropriate skills) and gain the equivalent "magical energy level". It can be stored for up to moon duration and mutliple sacrifice cannot stack, only the highest wound level counts (Five medium wounds bring only what one Medium wounds would bring, but a light wound sacrifice can be improved by a Heavy wound sacrifice).
It could Mysteries which could be initiated by a Blood Cult or self-initiated based on some Diedne information.
One suggestion would be to stay away from "Necessary condition: sacrifice", because it does not give any option to the character: to perform his magic, he needs to make sacrifice. I would reserve this kind of flaw to NPCs. I would find more interesting than Blood magic give him access to more powerful magic (or without blood magic his magic is weaker, like "Deleterious circumstances: without spilling blood") so he has the choice to be an average magus, with clean hands, who is okay in his field, but could be awesome if he was willing to embrace his dark heritage. Or he could be a repenting Blood magus (dark secret is an good flaw, or Plague with Supernatural entities if ghosts of previous sacrifice haunts him).
It can even lead to an interesting dilemna if the other players are facing an ennemy they cannot defeat, but the ex/reluctant Blood magus could defeat if is willing to sacrifice some humans...
Another flaws/virtues combo could Ghostly Warder being an ancestor who was practicing Blood magic and could initiate his descendant if he is willing to pay the price (then Plague with Supernatural entities has an initiation flaw is well suited).
A lot of the mystery cults have 'sacrifice' as part of their advancement. Verditi, for example, often go the route of chopping their own (left) foot off to mimic their founder and advance along their own mystery path.
If you do go with vis from sacrifice, it should take more than one person to generate vis, whether that be a large number of animals or a point of confidence and a human life...