It was just any other day, another testament to the mundane dreary lives of the living. No one was prepared for their coming, how could they be? They entered the bodies of the living, transforming them into blood thirsty monsters, all in an instant.
The players see a small shrine to the local nature deity just of the trail. Before they even approach it, they can smell the foul stench of rotting meat. If they inspect the shrine, they can see it has been desecrated by rotting organs in the last few days. There is no mistaking it for an obscure ritual, the organs are thrown everywhere, not left in specific places as in sacrifice.
If the players try to clean the shrine, they will soon find it has been boobytrapped to fling sharp splinters covered in the rotting gore in every direction. While only doing a few points of damage, they injured players will likely take sick soon unless they get medical attention.