The Blackmarrow Cult are a group of necromancers, led by Orryn the Black, trying to reanimate the skeleton of Thurvokun, a long-dead dragon. They are the main enemies encountered in Fang Lair.
The Fang Lair style is based on this group, and Crafting Motif 58: Fang Lair is a manual written by Orryn for how his cultists should dress in order to make "an impressive showing".
After reciting the incantations in Orryn's Notes, defeat Orryn the Black and Thurvokun without any party members being hit by Orryn's Wraith Thralls in Veteran Fang Lair.