The blueprint is acquired by the Heroes in Act III. It adds a strong 4X poison damage modifier and an increased critical chance of 6% to a weapon. The only downfalls are that the mod requires the use of a Stingray Tail which may be hard to find, and the weapon can get expensive to maintain.
"I developed a simple poison that can turn a sharp weapon into something that will make the undead spew out their own internal organs. A sight to see, believe you me. All you have to do is follow the guidelines below."