The Morrow Stone (Chronicles of Aesirium, Book 2)

Book Two of the Chronicles of Aesirium: On the world of Aerthos, the large community of Oldtown-Against-the-Wall has long been the refuge for the former citizens of the city of Aesirium, Utopian citadel of science and magic. But outside the defensive wall, the exiled townspeople have fallen under siege by undead monsters from the wild. Among them, however, a trio of young children seem to be key to the survival and evolution of this town. Can they learn to utilize their individual skills and talents in time? And what is the Morrow stone, what role does it play, and why do so many people seek it?