Duergar

As evil subterranean races go, the duergar are vastly underrated.  Think of this as an opportunity, though—since duergar haven’t been played out, you’re the one who gets to define them in your players’ minds.  In Pathfinder’s Golarion setting the duergar are almost tragic, doomed to toil but never succeed thanks to their deity’s nature.  But in most campaigns they’re also one of the few civilizing forces below the surface.  If devils have taught us anything, it’s that “evil you can and must deal with,” in the long run, is always way more interesting and adventure-complicating than “evil you must kill immediately.”  And there’s always the question of your world’s answer to how they got their magical powers…

Dwarves on the world of Feld are at a loss to explain how their cousins fell from grace, or how they developed their magical powers.  But the unseelie fey know—especially the size-changing spriggans...

The trundling duergar city of Spiral is part drill and part doomsday clock, boring a tunnel through the crust even as it counts down to some nefarious end.

Not all duergar remain belowground.  In the desert emirates to the south, gauze-mask-wearing duergar mingle with elven bounty hunters and human, azer, and jann slavers.  Their enlarge person ability especially helps them go toe-to-toe with the ogre conscripts employed by their human rivals.

—Pathfinder Bestiary 117

While we’re on the subject, evil dwarves in general haven’t gotten a lot of love since 1st Edition, where they were major players in the Dragonlance setting…though the Scarred Lands’s Charduni had the potential to be quite terrifying.