my first question would be: how often do you expect to get bitten? i ask this because unless this is going to come up often a character probably would be better off spending their refresh else where.
Not very often. Maybe every few games, and of course only when facing vampires.
A little background- I didn't come up with this during character creation. For years now I've been writing pieces of stories about a modern-day mage who ended up being a lot like Harry Dresden (imagine how annoyed I was when I read the books, heh). This was one of his little gimmicks. The idea is that magic can be very powerful, but all the time you spend practicing spellcraft takes away from kung-fu lessons, so he knew if a vamp got close to him, he'd need something fast with enough bang to get him out of trouble.
At this point, he's likely to be an NPC, unless I end up as a player in a group that specifically needs a wizard. Taking that into consideration, I can pretty much do whatever I want, and not worry too much unless one of the PCs wants to get one. It's mostly for this possibility, and my desire not to be a jerk GM with his annoying GMPC, and for the sake of interesting discussion, that I even bring it up.
I think if the power isn't game-breaking, and in concept it really isn't. it would be fine as an enchanted item slot. If I built it as a power, I wouldn't want to spend more than 2 refresh on it.
The next interesting question is: Story-wise, what makes this thing work? Even if it isn't game-breaking, how is it able to break the rules of how magic usually works? Is he privy to a secret technique of infusing wards into living beings? Does he owe a powerful being a favor for this? Does it come at some other cost?
