Secret under the Old Oak

Tosia was late again. She ran across the yard, dodging the neighbour's beige cat, until she finally reached the wicket where Michael was already waiting for her, leaning against his bike. - Well, at last! - he sighed with feigned anger, but his eyes were laughing. Tosia showed her tongue and adjusted her rucksack, from which protruded a torch and a notepad. - Come on, before it starts raining, we have to make it to the Old Oak! - she called out. They set off across fields and meadows that were already slightly yellowing from the August sun. The forest was their favourite place - quiet, full of rustling leaves, the smell of moss and... mysteries. The Old Oak stood at its edge, huge, with a hollow so big that Michael could hide all in it. - I heard that a fox lives in the hollow. - Tosia looked around carefully. - A fox? More like a dwarf, if you ask me! - Michal pulled a compass out of his pocket that his grandfather had given him. When they reached the tree, the wind began to rustle the leaves and the clouds grew thicker. Tosia leaned over and noticed strange marks on the bark - they resembled spirals and arrows. - Do you see these? - she whispered. Michael came closer. He touched the gouge with his fingers, and suddenly the roots of the oak tree trembled and began to split open. A blue light shimmered from within the tree, and a door-like circle appeared in the trunk. They both held their breath. A quiet whisper came from inside the portal, as if someone - or something - was inviting them inside. Tosia looked at Michael, and his eyes glowed with excitement and a little fear. The portal pulsed with light, and sounds began to emanate from the depths of the tree, as if the whole forest was playing an invisible melody. - Are we going in? - Tosia whispered. They both stretched out their hands and....