From publisher blurb:
The Monster Manual provides only one duergar (a CR 1 creature). Out of the Abyss supplies a few more variations, but any Rage of Demons campaign could use more options. The variations herein can provide you with a larger variety of duergar characters and enemies, ranging from CR 1 to CR 5.
Note that full stat blocks for the incomplete duergar from the appendices of Out of the Abyss are also included here, with just a few adjustments — that is, the darkhaft, the kavalrachni, and the Keeper of the Flame.