As the clock on the tower struck midnight, Maja closed the last book and looked at Tom, who was sitting opposite at a large oak table. The library where they were volunteering this summer was over a hundred years old - and had always been intriguing with its labyrinth of corridors, creaking staircases and shadows dancing across the walls. This time they stayed longer because they had to finish tidying up the book collection and help Mrs Sophie, who had been librarian since time immemorial. Charles, the third of their pack, was late again, but that didn't stop them from enjoying the atmosphere of the old building, beneath which supposedly stretched dungeons unknown to anyone. - Do you think these legends about tunnels under the library are true? - asked Maja quietly, looking around to see if the ghost of the age-old place was eavesdropping on them. - 'If only there was somewhere to go...' muttered Tom, stepping over the patterned tiles of the floor. Suddenly the door to the cellar creaked slightly. Charles stepped inside with a broad smile, brandishing something in his hand. - I got lost in the back room and... look what I found! - he triumphantly showed them a rusty key and a yellowed piece of paper. Maja quickly burned at the old legends she had read: "Only the chosen ones will open the door with the hidden rune". - 'I think that's the plan of the underworld... and the key to enter! - whispered Charles, and his eyes lit up in the glow of the lamp. A moment of hesitation and excitement was shared by all. They took the torches and the key and tentatively descended the stairs to the dark cellar. Behind a bookcase of dusty tomes, they found an iron door with an engraved rune, identical to the one on the plan. Tom inserted the key, which squeaked in the lock. The door opened with a groan, releasing a blast of cool air. In front of them stretched a narrow, dark corridor, leading deep into the ground.... The friends looked at each other, not knowing what awaited them in the darkness beneath the old library. One step - and the mystery was about to begin.