Mary Pope Osborne - Summer of the Sea Serpent

Book 31 in the Magic Tree House series (2004)
A novel by Mary Pope Osborne
In a LAND LOST IN MIST, in a time before time, there is a powerful Sword of Light. Now Merlin the Magician needs that sword for the good of all Camelot. But who can go on this dangerous mission? Merlin turns again to Jack and Annie--two modern kids with a lot of experience with history, mystery and magic--for help.