Warning: This post contains spoilers for season seven of Game of Thrones.
Following Sansa and Arya Stark’s reconcialiation at Winterfell, the army of the dead was shown emerging from the forest just beyond the Wall at Eastwatch-by-the-Sea. The Night King then flew in on the back of Viserion, who proceeded to blast blue fire all over the Wall.
“Whereas common dragons (if any dragon can truly be said to be common) breathe flame, ice dragons supposedly breathe cold, a chill so terrible that it can freeze a man solid in half a heartbeat,” the A Song of Ice and Fire companion book The World of Ice and Fire reads.
The Wall was originally built to defend the realm against the White Walkers, ice creatures who were created by the Children of the Forest in an effort to defend themselves against the First Men. It was also said to be imbued with magic to prevent the dead from crossing over.
However, considering the birth of Daenerys’ dragons was the event that reawoke magic in the world of Thrones following a long period of dormancy, it stands to reason that an undead dragon would be a powerful enough being to overcome the Wall’s wards.
“The Wall’s kept these things out for 8000 years. There’s no real reason that it can’t keep doing that unless something puts a hole in the Wall,” explained showrunner D.B. Weiss in HBO’s post-episode rundown. “There’s one thing on the board from the beginning that is now big enough to do that, and that’s a dragon…Winter is here and it’s all hitting the fan from all directions.”