Stories of the poison known as "gu" go back centuries in China. Some of these tales are laced with black magic and witchcraft, but the more recent legends are of women who poison traveling men for seduction, robbery, or revenge. The poison itself is made by combining the venom of various dangerous creatures in a unique way.
There are several methods for formulating the perfect gu poison, according to Xu Chunfu, an official in the imperial medical bureau, who wrote about gu in 1556. The most well-known recipe is to gather different kinds of the “five poisonous creatures” and place them in a jar to fight. This done on the day of the Dragon Boat Festival, which is the fifth day of the fifth lunar month. Some say that the jar is kept in darkness for up to the year. The body of the remaining venomous creature, which has eaten the others in the jar, will become the source of the lethal poison.
The idea is that the last creature will have incorporated all the different poisonous compounds in his own body. But the scary tales of gu were used to marginalize the Miao and Lingnan people of southern China. The dominant Han folk told tales of Miao and Lingnan women who were a danger to travelers from northern China because they had gu and were willing to use it. Read the history of the poison that never really existed at Atlas Obscura.
is mentioned in roughly 1000 B.C. in the Chinese classic:
the I Ching. (one of 64 chapters is devoted to it) We are talking 1500 years older than the oldest sources referenced in this article. So, it very well may have existed, or if a 'legend', it was around for at least that long. You can look this up yourself, it is Hexagram 18. Food for thought.