Kuba, Lena and Maks had been friends for years and spent every summer together in the town by the river. For as long as they could remember, the castle on the hill, although falling apart, had always attracted their attention. One July afternoon, as a storm began to chase onlookers away from the ruins, Lena noticed something strange - there was a small tunnel hiding behind one of the boulders. When the boys got closer, they noticed footprints leading into the darkness. - 'Someone came in here before us,' whispered Maks, glancing anxiously at Kuba. - Or something - added Kuba, trying to joke, but his heart was beating faster too. Despite their doubts, curiosity proved stronger. Armed with torches and Maks's phone, they decided to go inside. The tunnel was narrow, and it was getting colder by the minute. The echoes of their footsteps echoed uneasily off the stone walls. Suddenly, Lena stopped abruptly. - Did you hear that? - She asked, shining her torch deep into the corridor. For a moment there was silence, but after a while they heard a quiet murmur and indistinct voices. Kuba's torch flared; its light stopped on something that resembled a huge iron door with a mysterious symbol engraved on its surface. Kuba knelt down and noticed that someone had left an old, stained notebook on the ground. He opened it and read the first sentence, scribbled down quickly: "It is forbidden to wake up what sleeps on the other side." Suddenly, the door began to open slowly with a shrill creak. The three friends froze, holding hands and staring into the darkness that lurked on the other side of the door....