Three teenagers find a hidden entrance beneath an old castle ruin and step into a secret buried deep under the hill.
Kuba, Lena, and Maks had been coming to the river town together every summer for years, and the ruined castle on the hill was their favourite place to pretend they were explorers instead of bored teenagers with too much free time.
That afternoon, the sky had turned the colour of wet ash. Wind hissed through the broken walls, and the first drops of rain were already darkening the stones when Lena stopped beside a cluster of collapsed blocks.
"Wait," she said softly.
Tucked behind the fallen rock was a narrow opening, half hidden by roots and shadow. Kuba crouched, peering into it, and saw fresh footprints in the dust. Not old ones. Fresh enough that the edges were still sharp.
"Someone was here," Maks murmured.
Kuba tried to smile, but it came out thin. "Or something."
Nobody laughed.
The opening led into a tunnel so tight they had to turn sideways to get through. The air changed at once, turning cold and damp, carrying the smell of stone and something older, like rust and wet earth. Their torch beams wavered over walls cut by hand long ago, the light trembling every time a gust of wind reached them from behind.
They had gone only a few metres when Lena grabbed Kuba's sleeve and froze.
"Did you hear that?"
At first there was nothing but their breathing. Then, from somewhere deeper inside the tunnel, came a faint murmur. Not one voice. Several.
Maks lifted his phone, its weak light shaking in his hand, and it landed on a massive iron door set into the rock. A symbol had been carved into its surface: a circle split by a jagged line, as if someone had scratched it there in anger.
Kuba dropped to one knee and saw a notebook lying in the dust at the base of the door. The cover was stained and warped with age. He opened it with care, and the first line, written in a hurried hand, made his stomach tighten.
It is forbidden to wake what sleeps on the other side.
A deep metallic groan rolled through the tunnel.
The iron door was moving.