Fog shrouded the village of Ravnheim, lazily gliding over the roofs of the grass huts and wrapping around the long boats by the fjord's shore. Erik, the 15-year-old son of a blacksmith, had been unable to sit still since morning. After yesterday's tumultuous meeting in the long house, an atmosphere of mystery had been granted to everyone. The inhabitants were plagued by disturbing news - two oxen had disappeared during the night, and tracks had been found on the shore that not even the oldest hunter recognised. Freya, his 13-year-old sister, was sitting on the bough of an old oak tree, reading a faded map she had found in her father's attic a few days ago. Normally no one paid much attention to old swathes of paper, but this one was different. The parchment showed an island that no resident of Ravnheim knew, with the place marked in red and a runic inscription: "Only the brave shall reach the gates of Skallagrim". The rest of the village ignored the children's curiosities, but Erik felt a sudden shudder. Could the disappearance of animals and the strange map be linked? Freya suggested: "We will sail there tonight. We'll see what's hiding behind the fog." They devised a plan until nightfall, whispering by the light of the archer so their parents wouldn't hear them. When the first stars shone in the sky, they slipped out of the house, passing a sleeping guard at the marina. Into an old boat they packed a rope, a torch and a piece of dried meat. As they set sail, the fjord suddenly seemed boundless and alien to them. The fog thickened and a silence grew around them, as if the whole North Sea had held its breath. After an hour of slow paddling, the moonlight fell on something in the distance. The island, whose outline was barely visible above the water, looked like the toothy body of an unknown creature. Erik felt his heart beat faster. As they approached the shore, a piercing scream rang out - not human, not animal, as if the air had been torn apart by magic itself. Freya squeezed her brother's hand. Next to the boat, between the reeds, they saw something shiny - a fragment of a broken shield with an engraved symbol, identical to the one on the map. There was a rustling sound, as if something - or someone - was moving in the darkness on the island. The siblings looked at each other anxiously when suddenly....