Item from acotar-wings-and-ruin by
A poisonous herb that chokes the magic from a faerie's blood, leaving them mortal-weak and helpless.
Faebane is the mortal world's oldest defense against the fae — and Hybern's newest weapon of war. It grows only south of the Wall, in soils poisoned by old battlefields and long winters, and it is the only substance that can reliably weaken a faerie's power without killing them outright. A pinch in a drink, a smear on a blade, a cloud of dust in the air: each robs the fae of their gifts, leaving them stranded in their own frail bodies while the poison works.
A low, creeping plant with jagged, ash-grey leaves and clusters of tiny, pale yellow flowers that seem to wilt the air around them. When dried and ground, it becomes a fine, bitter-smelling powder the color of dried bone; when brewed, it steeps into a murky, green-tinged liquid that leaves a film on the tongue like cold iron.
Also known as: fae-kill, ash-weed, the bitter death, fae's bane