Nightmare Becomes Reality

Write a horror story about a nightmare that manifests in reality.

This prompt revolves around the spine-chilling concept of dreams bleeding into the waking world. What is the nightmare about, and how does it start affecting reality? Is it a monster of your own creation, or a relentless situation you cannot escape no matter how hard you try? How do the dream world and the real world intersect, and how do the characters cope with the terrifying turn of events?