One of my players has been running around for a while now getting blood samples (usually without their permission) from both PCs and notable NPCs (antagonists and allies). At first this was just so that he could give them negatives if he ever had to scry on them, but he has invested so much effort in this that he now wants to do more with it. I wont let him get Ganji Dolls (I don't like the idea of him being able to kill people half a city away), but I do want to give him something to do with the blood. So I created the following metamagic ... but I wanted to know what other people thought before I offered it to him as an option.
Bloody Spell (Metamagic)
You have learned how to use discarded parts of a target’s body to form a link between yourself and a target.
Benefit: When you cast a bloody spell you must have a small piece of the target’s body, either a hair, some skin, or some blood. A bloody spell cast in this way has a DC 2 higher than usual, gains a +5 bonus to spell penetration checks, and can be cast as a range of close, even if the spell is usually only deliverable at range touch. A bloody spell can only effect one creature, even if the spell would normally affect multiple creatures. If the target of a bloody spell manages to make the will save against it, any effect that they avoid by succeeding at the save (either the entire spell's effect or half the spells effect) is then reflected back at the original target (who can make any saves normally allowed by the spell to negate or reduce the effect).
Level Increase: +1 (a bloody spell uses up a spell slot one levels higher than the spell’s actual level.)
Having to collect the blood seems already like a price to pay, so I'd suggest not to make it a metamagic feat but a normal feat so it doesn't increase the level of the spell.
Some ideas about the benefits you could give for using the blood as a spell component/focus (I'd give a higher benefit if the blood is spent on the spell):
Increasing the DCs on the spells casted on the target.
Allowing a single metamagic feat among a list (enlarge, extend, etc.) to be used on a spell affecting the target without changing the level of the spell.
Allowing some spells to work within 1 mile on the target instead of their normal range (this could be similar to the Scar hex).
To add some flavor I'd make that any spell being cast this way would have a material/focus component even if the original spell didn't have one. Making the blood a material component or a focus depends on how do you want it to work and whether you think the blood should be spent or not. I wouldn't allow the player to get really powerful benefits if the blood isn't spent, as he could use it multiple times. But if you determine that the blood is spent on the spell I'd consider giving him some potent benefits.
I like your idea Rednal, but I am worried that if I let my blood obsessed player get a bonus like that, that all my other players with spells will start trying to steal blood or keep trying to get some of the blood my blood obsessed player has collected, and that ruins the unique feel his PC is starting to develop. Likewise I am worried about having it used up when he casts the spell because ... I didn't actually give him a definitive amount of blood to begin with and I would have to start keeping track of that kind of thing like a resource.
I might just offer him a feat like spell focus (blood) which would give him a +1 to DC and +2 to spell penetration if he has someone's blood. That would encourage him to keep getting blood, but wouldn't be that much more powerful than existing feats.