Said supernatural force in The Outsider is a creature the story commonly refers to by the Spanish moniker of El Cuco, but it's made clear that this beast is likely the inspiration for boogeyman-type myths across societies, languages, and belief systems.

In Flint City, Oklahoma, the mutilated and raped corpse of Frankie Peterson is found. Fingerprints and DNA at the crime scene as well as witness accounts all clearly indicate local sports coach Terrence Maitland as the killer, so detective Ralph Anderson orders a public arrest.

Holly Gibney (Cynthia Erivo), however, was able to stop the killing by daring Frank to shoot her. This allowed his conscience to overpower the influence of El Cuco, and gives Frank just enough autonomy to get himself bitten by a rattlesnake, and then when that does not kill him he kills himself with his gun.