Is blood thicker than magic?

During the second great war, waged against Gaia and her armies by the demigods of Camp Half-Blood, and in Harry Potter's sixth year at Hogwarts, when Lord Voldemort is steadily rising to power for the second time, two key demigods in the fight against Gaia disappear from Camp Half-Blood, waking up a few days later in a train compartment. A Hogwarts Express train compartment. Let the fun commence.