Lair of the Lantern Worm

From the introduction: Deep in the desert, an order of Heelan mystics practices a strange and lethal form of divination in a network of cliff-side caves. Brought from deep underground by an unnatural source, the water - toxic to Heelan - is a source of mystery to them. it is also home to one of the deadliest creatures of the underworld, a lantern worm.