SCP-939 is a Keter-class anomaly classified as a species of eyeless, red-skinned quadrupedal predators. In place of eyes they have only light-sensitive spots along the back, and their translucent red skin derives from a hemoglobin-like compound. Their most dangerous trait is the ability to reproduce the exact voices of previous victims to call in new prey. They also exhale an aerosolized amnestic (AMN-C227) that blocks short-term memory formation.
The heart of the horror is the betrayal of a trusted voice. That a familiar voice crying for help is in fact a trap makes the very sense of hearing suspect. How they acquire those voices is never explained, and it belongs to the kind of horror that renders what you hear impossible to believe.