From PathfinderWiki

Viperpool is the ruined remains of Arima in Avernus, the former domain of the deceased archdevil, Typhon. When the rest of Arima was lifted into the air to become the Promised Land of Barbatos, the remainder of the dark garden-realm was cast into twilight and its ruins and serpent-dens were overtaken by the clinging vegetation of the infernal duke Furcas. What was left behind by Barbatos is now a section of the Knight of Laurels' expanded domain.1