Weaveweed

A small, dense weed that grows thickly in the presence of magic, Weaveweed is a mystery to many scholars of The Academy - It seems to need neither light nor water nor any sort of food, growing in any place that is saturated with magic's weave.
The plant is a coiling deciduous vine, needing only shallow roots and with large blue leaves that curl in on themselves, taking the shape of small spirals.
Most notably, the leaves, if taken and brewed with a mixture of water, salt, and beholder blood, can be used to make a tea known as weavetea - a bitter concoction that renders anybody who drinks it entirely incapable of casting magic for as long as it is in their system.
Possession of weaveweed, weavetea, or any related products are considered to be a violation of one of the Laws of Magic, punishable by confiscation of goods and a stay in prison.