Gu poison

Gu poison is an extremely dangerous form of dark magic prepared via a special ritual.

The gu spirit within the poison can be left inactive until the poisoner chooses, making the poison useful for taking hostages. The spirit can feed off of the evil in the victim's heart and possess them. If the victim's heart is filled with wickedness, it can consume them even before the poisoner activates the spirit.

Gu poison can be removed by the poisoner or by a purification pill. However, purification pills can only be made by tier 4 alchemists, which prior to Feng Lian Wan's arrival, were purely theoretical.