Haunted Castle on Hallow's Eve is the thirtieth installment to the Magic Tree House series and the second of the Merlin Missions. This is also the first book where Jack and Annie have proven capable under the right circumstances of performing their own magic. In it, on Halloween, Jack and Annie are summoned by Merlin to retrieve the Diamond of Destiny to save Camelot. To help them, he sends young sorcerer Teddy who Jack and Annie had rescued from being turned into a dog earlier in their adventures. The three have to battle the Raven King who captures Teddy and is on the verge of defeating them when Jack manages to transform him into a baby raven and save the day. They then return Teddy to human form and return the Diamond to where it belongs before going home.