Mystery under the stairs

The rain was ringing against the large windows of primary school number seven and the corridors were almost empty. Most of the children stayed in the classrooms, waiting for the storm to pass. Kacper and Lena, two best friends from Class 5B, however, slipped out under the pretext of escorting their books to the library. Their favourite place? The stairs at the end of the old wing of the school, where no one had looked for a long time. It was there that echoes of old stories resounded - of hidden passages and secrets from when the school was still a palace. Kacper, always the first to seek adventure, noticed that today, through the dampness on the walls, something had changed. Just below the penultimate step, a small indentation could be seen, like a tiny door hidden in the shadows. - Lena, look! - he whispered excitedly, pointing to the place. The girl knelt down beside him, running her fingers over the slightly cracked wood. It seemed that the door was locked with a tiny, rusty key, but.... - Do you think this is the entrance to some kind of storage room? - Lena asked, a spark of curiosity shining in her eyes. Kacper put his ear to the door. Was it possible that he had heard a quiet murmur or whisper? Or was it just his imagination suggesting strange sounds amidst the thunder outside the window? The storm outside had taken a turn for the worse. Thunder tore through the sky and for a moment the light flickered. Suddenly Lena noticed that a key was dangling from Kacper's shoulder, hanging from a chain he had once found at school. - Let's try it with this! - she said, and her voice trembled with emotion. Kacper put the key into the lock. The lock squeaked and the smell of dust and old paper could be smelled in the air. The door swung open, revealing darkness and steps leading down.... - Shall we go in? - whispered Lena. At that moment, a cold breeze swept through the corridor and a quiet sound came from inside the stairs - as if someone had pulled a chain or moved in the shadows. Kacper looked at Lena and for a moment neither of them moved, staring expectantly at the black entrance....