Let's say it has to be the death of someone above the age of 30. They have to either volunteer or be a murderer. Since we're dealing with hypotheticals there is no way one could fake having a volunteer or murderer. Otherwise the ritual would not work.
The power received would be key knowledge that only the opposite side(s) have, given to those who perform the sacrifice.
There is no way of knowing what the information will be, only that it is very useful to them.
Would you be willing to use this weapon?
I like that the conflict and opposing sides are just assumed - it's like we're all warring over something serious enough to involve killing people.
Also, once someone kills a murderer, they become one. A clever leader would only need to find that first murderer.
PS: Yes. For the serious conflicts I think it's a no-brainer.