I decided early on that I wanted this to be a modern setting. I have a sensation that the mood I want to create will work better if it's against a backdrop of normal life. So one of the first questions I had to ask myself was this: Why is the emphasis going to be on hand to hand combat, in a world with guns and such?
This is my resolution to that. In order to harm many of the minions of the Great Beings, there needs to be a 'connection' between the injury befalling them, and the desire of someone to hurt them. The idea here is that a bullet fired from 100 yards away is just a tiny chunk of lead, and nothing like that can possibly hurt an evil demon thing. When a sword is swung, or a punch is thrown, however, not only is there physical damage of the weapon, but the attacker, with the desire to kill, is holding on to it. It's that desire to kill that makes the weapon into something that can hurt Demons and their spawn. This reliance on intention to hurt a GB spawn means also that they can't be hurt by accidents, traps, or environmental effects.
So far, I have three 'stages' of this in mind. At the weakest stage, it is just as I described- the attacker with the desire to kill has to be touching the weapon used, to make the connection and thus hurt the monster. More powerful, 'higher ranking' minions of the GB's can have a second stage- the harm must be delivered in a more intimate way; bare hands only. Even a spear or knife breaks the connection between the wound and the desire to kill. Avatars of the GB's themselves can have a third and highest stage- an intentional attack can only hurt them if the implement (knife, bullet, or fist) is soaked in the still-warm blood of the attacker. It is important to point out that the first stage of this resistance to harm can be given to Acolytes as well, as a per-instance spell which racks up their debt like any other. These stages overlap, as well. A being with the first stage of immunity could be hurt by a blood-soaked arrow, for example.
Hopefully, this makes two things plausible- 1.) Anyone who wants to combat the GB's and their minions had better know how to fight, though keeping a gun onhand to deal with lesser-Acolytes and non-magical minions is good too. 2.) Many of the magical abilities the monstrous minions and Acolytes get will revolve around making themselves more powerful in hand-to-hand combat, since it's ultimately their only weakness.
Ultimately, this is all a tool to make hand-to-hand combat a central theme in a modern setting, but I think it fits thematically with what I'm going for as well. My questions are this:
1.) Does it feel to 'contrived', or does it flow well with the mood the magic system sets up?
2.) Are they any unintended game-breaking consequences of this set-up? I haven't yet evaluated it too much in conjunction with the rest of the game.
And of course,
3.) Does it seem 'cool' and make the game world a more interesting one to play in?

So you can't use a bullet except in the third level of attack? That is, the only way to use bullets is to soak them in your blood (or shoot through your own hand, or whatever), and such an attack is powerful enough to break through every level of immunity?

Right. A bullet isn't going to hurt any creatures with these sorts of immunities, unless it is soaked in the warm blood of the attacker. The bullet loses the effect to break the immunities once the blood on it cools- no advance preparation. Heating the blood artificially won't work.
You're also right to say that such an attack would be powerful enough to break every level of immunity.
Of course, bullets will kill non-magical people (like the PCs) just fine.

My initial thought on this is that not only does this fit in extremely well with the rest of the setting you have described in other recent threads, but also... these are 3 different stages of magical attack.

In fact, the third stage is an out-and-out blood sacrifice.

This means that within your setting, in order to harm the more powerful minions of a Great Being, you have to use magic, thereby placing yourself within the power of another - or the same - Great Being.

This has the potential to be a hose-job of massive proportions - not that that is necesssarily a bad thing!

Suggestion: don't make these attacks a requirement for dealing with these opponents, but award a significant bonus for thier use - this reinforces the Difficult Choice between "purity" and "power".

One question - should characters be aware that these actions are magic? I'm quite taken by the idea of characters being aware of the power of these techniques, and that they are magical to some extent, without being clear about the source of that power.

Useful?

I'm concieving of it a little differently than you are- to put it in the terms you're using, these actions that can hurt demons could be thought of as 'Good Magic', and isn't tied into deals with the GB's. The subtle point here is that GB magic is based on Debt, and human 'magic' is based on Sacrifice. I think I'd like to retain the notion of spilling your own blood to slay the monster as the pure alternative to magic, not as another way to succumb to temptation.

One question - should characters be aware that these actions are magic?

Uccisore wrote: The subtle point here is that GB magic is based on Debt, and human 'magic' is based on Sacrifice. I think I'd like to retain the notion of spilling your own blood to slay the monster as the pure alternative to magic, not as another way to succumb to temptation.

This is cool, only thought is that the level of Sacrifice may have to be more severe, otherwise there is no incentive to use magic - and that would be a shame

definitely come up with some more ways in which the competing themes of Debt and Sacrifice work with and against each other.
maybe being in a state of mind that's emotionally charged and very personally invested (like fighting demons) gives humans a kind of energy or resonance that makes demons closer to terrestrial matter. physical self-sacrifice for the sake of driving away demons could offer an edge in attacking and hurting demons, like you've illustrated, but maybe some kind of luck-based or even obviously supernatural effect could protect someone whose friend or loved one is desperately trying to protect them from demons.

ex. someone who puts him/herself in harm's way, facing the wrath of a demon, to protect another person could provide some kind of edge or mystical protection to the person being protected or what have you.

also, maybe tearing up one's own flesh could give system bonuses to finding and outwitting demons, as well as in fighting them. along with this, maybe you could sacrifice yourself (a bit less literally) as a diversion or human shield for someone you care about or love and then one of you (either the martyr or the quarry) could get bonuses. the martyr could get bonuses to ensure the success of this protection, or maybe the quarry could get bonuses to ensure good fortune and a speedy escape.
[by the way, i think you'd love the GameCube game Eternal Darkness: Sanity's Requiem. It has a stronger Lovecraftian feel than the magic system you've been describing, but it has a fair bit in common as far as flavor and some of the ideas.]

Enternal Darkness rocks, I played it back when it first came out. :)
As far as opening up what people can do with self-sacrifice, it ultimately comes down to a choice- do I want to reward people for not learning magic by giving them this alternate 'good guy magic' that is just as fun, deep, and interesting, ar the cost of watering down the temptation of learning evil magic in the first place,
or,
Do I want magic to be solely an evil thing, and these weaknesses we're talking about to be expressed as limitations to the GB's power, and not powers of the humans themselves? The trade off here, is that I'm not giving good-guy characters any cool supernatural story elements to play with.
I'm definately leaning towards the latter. Let me use the vampire example again- the fact that they can be hurt by a stake through the heart isn't a power that human have- the power is in the vampire, who can't be hurt by anything else. It's not as though humans have special wood-based powers in a setting with vampires.

with this, maybe you could sacrifice yourself (a bit less literally) as a diversion or human shield for someone you care about or love and then one of you (either the martyr or the quarry) could get bonuses.

I've not read any of your other posts so I'm commenting purely on this one.

Uccisore wrote:
1.) Does it feel to 'contrived', or does it flow well with the mood the magic system sets up?
2.) Are they any unintended game-breaking consequences of this set-up? I haven't yet evaluated it too much in conjunction with the rest of the game.
And of course,
3.) Does it seem 'cool' and make the game world a more interesting one to play in?

Depending upon how all of this is spelled out in the rules, I would be most definitely interested in this sort of rpg. I love the HTH aspect in the modern world. I have always had a problem trying to justify a modern supernatural investigator trapsing around the big city armed to the teeth with .45's and M-16's, and you worked around that issue in a way that I have not heard of before. Kudos!

Thanks for all the responses, combat-specific rules will probably be the part I post next.