From publisher blurb:
Blackdyrge's Bestiary: Terrors of the Under Realm has been completely revised and updated! The new version features all the old monsters in a new stat block format, plus an additional monster - the horrific, insectoid writher. Each monster includes an introduction by Blackdyrge himself, a beautiful new stat block, information on tactics, treasure, lairs, and much more!
Blackdyrge's Bestiary: Terrors of the Under Realm contains 30 pages of new d20 material, which includes:
A fiendish insectiod marauder with a taste for blood
A toxic troglodyte with a venomous bite
An undead choker with an appetite for murder and the flesh of the living
An earth mephit whose villainous deeds have garnered him a patron of truly diabolical proportions
A dragon-blooded behir that swallowed an entire village of deep gnomes
An otyugh so huge that only a literal sea of garbage could hide its odiferous existence.
New OGL material includes:
The venomous template.
Two new monsters: the writher and the rotwasp swarm
Stonespite, an intelligent and evil warhammer with the ability to turn victims to stone