Sympathizers join a cult... However, before they are allowed to join, they have to take part in a blood sacrifice.
Initially no details regarding the type of blood sacrifice are given.
They travel and meet up at the agreed location. Soon they find a letter informing them about the details of the blood sacrifice: they need to sacrifice themselves.
You might think; what was the purpose of finding the letter; terrorize them with fear or is it indeed the truth?
Whatever it is, they will have to complete all orders if they want a chance to even try and get out alive...