This is The Poltergeist of Baztán Valley by Sascha Schmidt, a mythical creature horror story for sleep from the Salus Omnium archive.

In 1937, during the Spanish Civil War, a converted textile mill in Spain’s Baztán Valley becomes the center of a poltergeist incident involving falling stones, unnatural cold, and bruised girls in an attic dormitory. What begins as folklore becomes a hidden biological reality: a distributed organism seeded through salvaged materials, feeding through the building, its water system, and the bodies trapped inside it.

Recovered reports and sealed field notes trace how Salus Omnium personnel attempt to interrupt the infestation by flooding the mill, burning contaminated objects, relocating trigger individuals, and preparing the structure for destruction. The horror is not only the creature in the walls, but  ...