Grim Hollow: The Monster Grimoire, The Darkfeather pg. 80
Poison
Varies Divine, Necrotic
Creatures whos wounds are exposed to this venom by a creature, object, or weapon must succeed on a DC 12 Constitution Saving Throw, taking 18 (4d8) poison damage on a failed save, or half as much damage on a successful save. If the poison damage reduces the target to 0 hit points, the target is stable but poisoned for 1 hour, even after regaining hit points, and is paralyzed while poisoned in this way.
Sandwiched between the south of Alkhesta and the north of Varisia there is a great scar from the 2nd plague called The Necrotizing Fetter. From this cursed and rotted land a dark creature mimicking the appearance of a plague doctor has risen and begun hunting within the region. Villages infected with plague and blight see these imposters preying upon their sick and weak. The Darkfeather, as they have been called is a fairly new creature, and as such it's venom stored in it's beak, is very difficult to find. A dose of Darkfeather Venom can be crafted into a Potion of Necrotizing requiring proficiency with a Poisoner's Kit, 4 hours of work and a successful DC 12 Poisoner's Kit (Intelligence) Check to craft.
Cost: 650 gp
Weight: .4 lbs.