Chapotraphouse

the long night of the show. It doesn't happen naturally or magically from the white walkers, it's a blacked out sky from a massive meteor like the one that killed the dinosaurs. It already happened before is how we know, the original long night coincides with the Bloodstone Emperor calling down a meteor and then worshipping it in the World of Ice and Fire, it's how the Great Empire of the Dawn was destroyed (the proto-Valyrian dragon riding empire). The Children of the Forest also call down the Hammer of the Waters and split the arm of Dorne.