In the grand history behind A Song of Ice and Fire, one event hangs heavy. It's a generational event that defined the lives of every person in Westeros. The Long Night was an inexplicable decades-long winter season that allowed the undead White Walkers to invade the continent. It could have wiped out humanity. Luckily, all living things banded together to fight the Army of the Dead and won. After they drove them out, Bran the Builder led the effort to keep them out.