Tephra Island is the largest of the Surveyor Islands, and the only one with significant inhabitation. It is a volcanic island, with the volcano being the Mountain of Ashes, part of the chain that extends from the Elder Mountain to the New Volcano. The island has one town, Fireburg, and two smaller villages - Fireberg on the Mountain of Ashes, and Firebog on the western edge of the island. In the Golden Age, Tephra Island was a nature preserve, excepting an international facility in the northeast where high-energy magic research was carried out sometime around the Catastrophe.
The inhabitants of the island are mostly followers of a religion worshipping fire. The religion is thought to have arisen from the worship of the Shadhavar goddess En-Bequ'avar, which was established on Equus during the Golden Age, a period in which cultural contact with the Dread Dimensions increased greatly. Fire-based magic is cultivated as a sacred art by the clergy of the faith. Water is scarce on the island, and is won from hydrated minerals and cloud herding. There is some trade, but the inhabitants seem to value self-sufficiency. Glass, gemstones, and magical items are exports. Luxuries and manufactured goods are the main imports, but the scale of trade is, again, small.