Book IV of Raven, Swordsmistress of Chaos. The unthinkable had happened.  The Wall of Thoughts, barrier against the unknown had been breached, unleashing a dark force into the world – the Ghost Lords of the Uthganaar, evil creatures who had united the Northern tribes of the Worldheart and were set to conquer the world.  Only Raven could stop them.  Together with Spellbinder and the valiant tribesman Silver, she made the perilous journey to the mysterious Obsidian Tower to learn the secret of the Ghost Lords and their invincible yellow sword.