John Bierce The Lost City of Ithos

Half a millennium ago, the sphinx Kanderon Crux and her allies banished the city of Imperial Ithos from the world of Anastis, in a desperate attempt to defeat the Ithonian Empire. Her dread weapon, the Exile Splinter, even erased the memory of its location from the universe. Now it's returning, bringing the Exile Splinter back with it. The great powers of the continent are desperately hunting for the site of the lost city, knowledge lost even to Kanderon herself. None know what ancient Ithonian weapons and enchantments might still be found in the ruins, but even the Exile Splinter alone would be a prize justifying war. Hugh and his friends find themselves dragged along on the search, where they'll face enemy warlocks, sea monsters, liches, unnatural storms, and even a man-eating tiger. There's something they're not being told about the lost city, however. Something that has even Kanderon and the other great powers terrified.