All of her skills were at full display during that scene. Her stealth-like movements, which even surprised her dearest brother John Snow, now a Targaryen. She jumped the Night King, surprising everyone, even his generals. For a moment he caught her. Holding her throat in one hand and her hand in another, she did something only a seasoned warrior or assassin would do. She made sure to use what was left, her other hand. In what will probably become one of the most famous witches of hand in history (some even compared it to Jordans switching hands antics) she let the dagger fall into her right hand and stabbed the Night King in his undead stomach. He turned to ice, much like the rest of his first-class warriors. The rest of his army returned to being dead once again, hopefully for the last time.
She saved her brother Bran, or as he would call himself now, the Three-eyed Raven as well, seeing Theon formerly of the House of Greyjoy, fall in an honorable fashion, redeeming himself for all the bad things he did to the house of Stark and erasing the memories of himself as the pitiful creature that was Reek.