Answer:
In The Lost Hero, Leo, Jason, and Piper find out that King Midas came back from the dead and has a golden touch. Piper's bag pack gets turned into gold, and then he turned Piper into gold too. They find out that the Hunters of Artemis were looking for Percy Jackson. Upon meeting the Hunters, they find out Thalia (the lieutenant of the Hunters) is Jason's sister. They also find out about their mother and that she left Jason, and gave him to Hera, as appeasement. Then Thalia offers to take them to the wind god.
hope it helps, i love heroes of olympus.