In the world’s oldest role-playing game, derros have been reliable, flexible underground threat: subterranean savages similar to morlocks, degenerate dwarves, and dwarf-human crossbreeds. Pathfinder has reconceived them as degenerate former fey—a nice retouch that certainly works for the Golarion setting (especially since duergar, morlocks, and dark folk have the degenerate dwarf, human, and halfling ground covered). Whatever race you decide derros are, you at least know what’s in their heads: madness. Every encounter with derros should be a mixture of horror and pity—an encounter with creatures trapped by their own insane logic and evil urges.
A derro scientist wishes to conquer his fear of the sun by studying the fears of his victims. While stealthy at first, he and his juju zombie assistants eventually abduct enough citizens that the human authorities begin to investigate. Meanwhile, the emotions his experiments provoke have drawn the attention of a chaos beast and several bugbears.
The dwarf prince Lugar seeks to wed the human crown knight Fianna. The dwarven clans are in uproar over the affair, but won’t say why, even to Lugar. Fianna’s nation is bemused but wary, and her liege will be insulted if the marriage is stalled. Only the dwarven elders know the truth behind their obstinacy: that every child of a dwarf-human marriage has grown into a mad derro. Meanwhile, as the wedding plans and the political negotiations drag on, a series of strange fungal poisonings threatens to send the situation into further chaos.
A pech sage thinks he can cure the derro race of their madness through a combination of ancient fey magic and divine miracles. Even as he tentatively begins to share his theories, powerful derro savants move to thwart him, convinced that they can use the same magic to permanently correct the insanity of the misguided pech and svirfneblin apostates.
James Jacobs delivers plenty more on derros in Classic Horrors Revisited. Also, if you can find it, check out Roger Moore’s legacies of the Suel Imperium in Dragon Magazine 241.
A victim of my Florida vacation, this entry is finally up! Hooray!