[WARNING: THIS ARTICLE CONTAINS SPOILERS FOR THE GAME OF TRONES SEASON 7 FINALE]
Besides all the huge reveals in the Game of Thrones season seven finale, we also got to see the White Walkers and the Night King tear down the Wall and start their invasion of the north. The threat of the whites has been teased ever since the first season, and the showrunners have revealed that the finale had been planned years ahead.
During an inside look of the episode (via ScreenRant), David Benioff and D.B. Weiss explain:
"We tried to contrast to contrast the various season endings so they don't feel too similar. So last season we had a pretty triumphant ending with Dany finally sailing west towards Westeros… This one is definitely more horrific. For many years now we've known this would be the ending of the penultimate season. The Wall's kept these things out for 8000 years. There's no real reason why it can't keep doing that unless something puts a hole in the wall. There's one thing on the board from the beginning that is now big enough to do that, and that's a dragon. They just start to suggest itself as a logical way forward."
They also explain the complexities of trying to pull off the whites' invasion onscreen. They say:
"We write ‘and then the wall comes tumbling down' and it's really easy to type those words. It's really hard for them to make it look good. It needed to be a thing you go out of the season on."
Any seasoned audience member would expect something as terrible as this to happen in the show because of the dramatic principle of Chekhov's gun. It's the rule that goes: "If you say in the first chapter that there is a rifle hanging on the wall, in the second or third chapter it absolutely must go off. If it's not going to be fired, it shouldn't be hanging there."
The rule is the reason why we're sure that the white walkers will attack. It makes us sure that Thanos will gather all the Inifnity Stones to complete the Infinity Gauntlet; and it makes us sure that the Prime Minister in the first episode of Black Mirror [SPOILERS] will bang that pig. You don't present a looming threat without giving audiences a pay-off in the future.
As of now, season seven of Game of Thrones is over, but we can expect the last season to arrive either in 2018 or 2019.
See Also: Game Of Thrones 8: Incestuous Romance Between Jon And Dany Will Be Addressed