The secret of the winter sock

On Christmas Eve morning, Hania hears a whisper from her sock. She finds a key and a secret door by the Christmas tree. What lurks on the other side?

On Christmas Eve morning, Hania got up very early. Outside the window the snow was creaking and the lights were twinkling. The living room smelled of gingerbread and oranges. The Christmas tree stood straight up like a silent sentinel. Hania's red sock hung from the branch. It was heavy and hopped strangely slightly.
Hania walked over, touched the sock and froze. A very quiet dinging rang inside. - Mum, she's saying something! - whispered Hania. Mum was stirring the borscht and listening to the radio. - It's probably just candy - she smiled. But the sock whispered like a breeze in the keyhole. - Hania... help... - sounded a whisper from inside.
Hania slid her hand in and felt a small envelope. On the envelope was the mark of a silver bell. Inside lay a note and a tiny key. The inscription said: Open the door at the Christmas tree, at dusk. Hania looked at the Christmas tree and noticed a small door. They were hidden just above the floor, behind a bauble. Three shiny flakes fell from the key. The clock in the kitchen said bim-bom loudly. Outside, the winter twilight was slowly falling. Hania took a breath and slid the key into the lock. Something on the other side rustled quietly. The door vibrated and slowly began to swing open....