The Citadel of Flame

Publisher blurb:

Once the home to a cult preaching self-sacrifice by immolation in order to achieve paradise, the Citadel of Flame in Qadira's Meraz Desert was presumably wiped out decades ago after Sarenrae dervishes led a crusade to end its morbid message. An enormous sandstorm swallowed both the citadel and the army attacking it, burying both beneath hundreds of feet of sand. Thought lost forever, the citadel's rediscovery prompts the Pathfinder Society to send you deep in the unforgiving Meraz to finally close the book on Qadira's most mysterious cult.