Right now, I'm writing a session or two for a Dark Sun game that I'm running. The PC's are going through an abandoned section of a city that's underground that they found after falling through a sand-sink into a cavern. The first part that they go through is a manor that once housed a mad sorcerer-scientist, who tortured and experimented on people and made flesh golems and flesh constructs.
The surrounding city was allotted to him by the Sorcerer-King of the city has his permitted "hunting grounds", where he kept everyone inside with the use of walls and flesh construct guards, and regular kidnapped people and used them for his experiments.
How do I make these sessions of the game as scary and as terrifying as possible?