Nightshade

"The name and shape of this plant are known to all; long understood to be one of the more potent components in many poisons, the average person keeps his distance from the bright purple flowers nestled among dark leaves. It is exceedingly effective as a pure poison, but can also be combined with other compounds to stiffen joints as well. It is thus favored amongst those who wish to disable their opponents in battle, and can be found coating the blades of many of the more unsavory characters in these lands"
Bonus Fact: Fairies are violently allergic to this