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This sounds a bit complicated. I was thinking of making it work like in the vanilla game, you click on the body a menu pops up asking you if you want to feast. I'll try figuring out both ways.
I think it would work either way, you need to test which way is easiest to link with the power and a racial check.
You may also want to think about StoryManager ... (I haven't thought this all the way through, but it is what struck me when I read your idea):
1. There is an SM Event for when an NPC dies and when it has been killed by the PC (and other conditions, so race of PC, race of NPC and so on).
2. You can get that event to start a Repeatable Quest
3. That Quest can be used to give the NPC a food item (or a menu item, to be fair, I think)
4. Somehow, using Quest Stages, you should be able to figure if the player has eaten the item generated and set via script the Power he acquired and how long for etc
5. If he does not eat, then the Quest would need to "finish" though ... and I'm not sure how you make that bit work.
Still ... if you can figure out the last bit, then doing it via SM and a Quest may give you an easier time (because it would "attach" to any NPC, and the exact NPC reference would be auto-passed to the "Quest"). And it may help make it easier to add more depth to your system (so, Elf corpses give longer duration power, or a different power, than Human ... or whatever).