Magic Tree House Merlin Missions #3 Summer of The Sea Serpent

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 Caemlot.  He turns again to Jack and Annie for help.  With only a rhyme to lead them, Jack and Annie set out on a journey filled with fabulous creatures.  But can they solve the ancient riddle before night falls?