From publisher blurb:
Learn about the Undercities of Aventyr’s duergar—the stubborn dvergr. First of the dwarves and heir to their true name, these xenophobes close off the Underworld from the surface realm, sealing away the endless caves from the world above.
In Underworld Races: Dvergr you’ll find:
great new mundane items like deadly weaponized picks, anchoring bolts, and explosive ambersticks!
the underminer archetype—Underworld cavaliers unlike any other mounted warrior!
new spells like find the surface and superior collapse!
new feats like Underfighter and Dvergr Pickman!
new magic items like the goggles of undersight!
new favored class options!
a powerful, memorable rendition of the standard duergar sure to enhance your Pathfinder game!