O Estranho Habitante: The Strange Inhabitant

In 1200, in the castle of Almourol, and ancient priest gives, a sacred relic to a monk, to hide it from the templar order, he then travels to an ancient forest, to stash it beneath an elder tree, but something also wants the relic, and searches for it, 814 years later, a group of teenagers goes on picnic, and they find the relic, it's just a matter of time, till the strange inhabitant finds them...