MURDER HOBOS
There's a good reason people on the frontier fear strangers to their village, and not all dark gods needs their sacrifices to be upon altars. A band of low level adventurers wander from village to village with only murder on their minds, but its not as random as it appears. In every case, the bodies are hacked apart and thrown down wells. The villages aren't chosen just for their remoteness. The villages all sit upon ancient locations of power, long forgotten by the current lords of the border kingdoms. The blood seeps down through the well water, deep into the earth where these powerful stones drink in the life of the dead.
Are they summoning demons? Awakening an ancient evil? Gaining earthly power? None of the above. These murderers believe are working toward glory in the next life, sending their sacrifices ahead of them to the great beyond where these poor villages will serve as their slaves in hell. Are they right or just insane?