Shadow of Death by Heather Graham

You can cut off the head of the snake, but another will emerge.

When two hikers go missing within a series of daunting caves outside of Denver, Colorado, FDLE special agent Amy Larson and her partner, FBI special agent Hunter Forrest, have good reason to suspect foul play. The pair of hikers are only the latest to vanish after a rash of disappearances that’s left local law enforcement stumped. But in searching the dank caverns near the Arkansas River, the agents aren’t prepared for the horror they uncover: a muddy pit littered with corpses. Covered in bite marks. Made by human teeth.
When a tiny toy horse is found on the scene, Amy and Hunter recognize the calling card. They’ll have to move quickly before the already sizable body count grows. Their investigation soon draws them down the rabbit hole of a dangerous cult with a sinister mandate—one that involves human sacrifices. Anything to further their twisted cause. But when more people go missing, it becomes clear the cult’s reach extends beyond state lines, leading Amy and Hunter dee