Killweed is a herb found only in Evil Forests. It looks like the Taintweed in Thuamcraft 2. When added to a potion,  it adds the "Instant harm" effect for 23 HP. It also has effects if you move it into a new, pure forest. If in any Non-Evil forest, it will slowly turn it into an Evil forest, unless it and all other Killweeds in that area.