Christian Heldbo Wienberg

Nis, a smart Danish kid, studies Latin to research Templar history on Bjornholm. His friends Mathias and Kathrine reluctantly join his treasure hunt. With help from his sister, they uncover two rival Templar factions and face peril.

14 y.o. girl's dad is kidnapped by men, who are interested in the Knights Templar treasure. With 3 friends she sails from Bornholm, Denmark, to Gotland, Sweden, to free her dad and perhaps even find the treasure.

The snake crown is stolen (or taken back) from the Knight Templars by the earth worshippers of Crete; the three kids try to retrieve it on behalf of the Knight Templars.