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?
How are you supposed to fake a murderer? They are or they aren't.
I see you've now added the additional detail that the murderer holds no side but their own. Your hypothetical fake universe is getting a lot faker. Usually sides consist of teams.
If my team sacrifices a murderer against their will, whoever murders the sacrifice becomes a murderer. If they share their info with the rest of the team we also have another murderer to sacrifice to get more info. Maybe if three people hold the knife we now have three extra murderers.