Xidao trade spires

From PathfinderWiki

The Xidao trade spires are tall spires that jut from shallow, coral reefs and atolls across the Xidao Gulf. These stone obelisks feature dozens of small shelves and niches on which offerings like pottery, metals, glass, weapons and even terrestrial foods are left for the citizens of Xidao who live below. Those who make such offerings are left in peace to reap the bounty of the Xidao Gulf, those who offer nothing tend to find that the ocean can be more treacherous than expected.1