Brimstone (Stargate Atlantis, #15)

The Stargate Atlantis team discover a city on a moon that's about to plunge into its own sun. But the city which looked as if it had been abandoned turns out to be inhabited by descendants of the Ancients who have fallen into decadence and debauchery. Faced with a dissolute society disinterested in their own fate and unable to escape, the team must fight their way free before being plunged into a fiery death.