One of the places mentioned by Nie Li in his questions to Shen Xiu about people and places outside of Glory City. She had heard of it but never seen it. The Toxic Forest lies somewhere to the west of Glory City and the St. Ancestral Mountain Range.
It is presumed that this is one of the places visited by Nie Li during his many travels before coming back in time. The people there survived countless attacks from poisonous demon beasts and have thus developed high level skills in alchemy.