https://www.cnet.com/news/hbo-kept-aryas-biggest-game-of-thrones-moment-secret-for-years/

Then, the highlight: with a mighty scream and some fancy knife work, Arya Stark (Maisie Williams), the trained assassin, plunged a Valyrian steel dagger into the Night King, disintegrating him and all of his deadly creations. 

If I knew that was going to happen, I would probably be blurting it out everywhere, unable to contain myself. Game of Thrones showrunners David Benioff and D.B. Weiss appeared in an Inside the Episode documentary to reveal they've been keeping Arya's moment secret for three years. Three long, long years.