Come visit me at Ragnarok. They got soaked in Spain

Few modern teenagers have had to fulfill the request of the ancient Scandinavian god Odin and confront another Scandinavian deity, Loki. And then the capricious bridge Bifrost is mocking, and the roast boar's head at the feast in Valhalla is winking — it’s very difficult to prevent the end of the world under such conditions. But the twins Vlad and Danil from the story «Come visit me at Ragnarok» managed to do it, although the sardonic Loki said: «And you seriously think that people need gods to bring about the end of the world?»
In the story «They Got Soaked in Spain», the task for the twins is simpler: just find the escaped head of a ghost who was once a medieval knight. «Got soaked» means not «killed», but rather «got wet», because throughout the story in «sunny» Spain, it is raining, which does not please the brothers at all. And if Spain had always been rainy, how would its history have changed? The author proposes a completely different version of historical events than what actually happened.
The book is recommended for middle school-aged children.