The Season 8 premiere ended with the reveal via the late Lord Umber that Last Hearth had been taken already, and Tormund and Co. were able to give the folks at Winterfell at least a bit of a head’s up about when the dead would arrive.
Game of Thrones has been building to the conflict between the living and the dead from the very first scene of the very first episode, and it’s finally coming on a truly grand scale with the army of the dead marching south, led by the White Walkers with the Night King presumably at the head, with undead Viserion as his steed.