Spellbane

Spellbane was a type of plant often used as a herb for its magic-nullification properties,

Ecology
The spellbane plant was only known to grow in Cormyr. It clung to rocks in areas of deep shade, such as in clefts, in piles of stones in forests, and in the mouths of caves.

Effects
A living creature that ate a piece of spellbane lost all ability to cast spells for a period of 3–12 hours, though it did not impede their ability to use magical items. For the same period, the creature gained a resistance to magic, or an enhanced resistance if they already possessed such a thing. The degree of resistance depended on the amount consumed, from a sprig or leaf, to a whole plant, to a vast quantity, though it had an upper limit.

Availability
Around 1369, Hanno Minstrelsong of Hanno's Herbs and Medicines in Wheloon, Cormyr, sold spellbane. It was the rarest and most expensive of his stock. He usually had only one small live plant or a small keg of dried leaves in stock. The plant would cost at least 100 gp, while a piece was upwards of 40 gp, and one dried leaf was 25 gp.