Jorah Mormont (to Daenerys): The Dothraki have two things in abundance: Grass and horses. People can’t live on grass. In the Shadow Lands beyond Asshai, they say there are fields of ghost grass with stalks as pale as milk that glow in the night. It murders all other grass. The Dothraki believe that one day it will cover everything – that’s the way the world will end.

From Game of Thrones – Season 1 Episode 2: ‘The Kingsroad’ (1×02)