Essence of Nightshade

A poisonous substance which Cersei acquires from the Maester in Kings Landing prior to the invasion in Episode 9 of Season 2. The Maester informs her that 3 drops will put a person into a deep, dreamless sleep, and that 10 drops will do more. Cersei interrupts him before he can conclude that 10 drops will quietly kill a person, but the assumption is made.