Wolfsbane

Wolfsbane. A rare substance. This is no common assassin.|[[Tywin Lannister|A Man Without Honor}}

Wolfsbane is a lethal poison obtained from a plant of the same name. Darts dipped in it may kill a grown man quickly.

Season 2
Jaqen H'ghar murders Ser Amory Lorch using a poisonous dart dipped in wolfsbane. Tywin Lannister identifies the poison from the smell of the dart.

In the books
In the A Song of Ice and Fire books wolfsbane is mentioned as a type of poison ut is never employed in the narrative. Jaqen H'ghar's second victim, Weese, is seemingly killed when his dog tears out his throat.