Ghost grass

Ghost grass is a type of vegetation that exists on the continent of Essos. It is found in the Shadow Lands beyond Asshai. Ghost grass is pale in color and glows in the moonlight. It kills other types of grass.

Season 1
Ser Jorah Mormont tells Daenerys Targaryen that ghost grass is native to the Shadow Lands. The Dothraki believe that one day ghost grass will fill the entire world, killing all other types of vegetation, and that is how the world will end.

In the books
In the Song of Ice and Fire novels, ghost grass is described similarly to the TV series. The map collection called Land of Ice and Fire shows ghost grass growing in vast quantities all around the mountains of the Shadow Lands, extending down to Asshai and east along the shores of the Saffron Strait, as well as north-west towards the borders of Yi Ti.