Overview

The "Gu" was a common poison in ancient China. It was made by placing venomous animals into a large pot together. These animals included snakes, toads , scorpions, spider, and centipedes. After a period of time passed, the pot was opened and the surviving animal was deemed the most venomous or poisonous. It was subsequently ground up and used as a poison.
Ref: China's Golden Age: Everyday Life in the Tang Dynasty by Charles D. Benn, Page 197, Oxford University Press 2004