From PathfinderWiki

Arima is the former domain of the deceased archdevil Typhon, the former ruler of Avernus. Once a garden of darkness, snakes, and perverse delights, part of Arima was lifted into the air to become the floating domain of the layer's new ruler, Barbatos, becoming the Promised Land. The remainder was overrun by the deadly plants of the neighboring Hanging Marches, becoming known as Viperpool.1