Plenty of people have tried to kill Game of Thrones's seemingly invincible Night King, but none have succeeded — until now. That's right: the Night King is dead, at the hands of Arya Stark. As the battle ramps up in intensity and the Night King finally breaches the godswood at Winterfell, it seems like all might be lost, until Arya leaps onto the Night King from behind, flips her dagger from one hand to the other, and drives it into a gap in his armor. The dagger that Arya uses in her huge hero moment actually has an important backstory to it that adds even more significance to the scene.
Arya's dagger, made of Valryian steel, has actually been around since the second episode of the first season, when it's wielded by an assassin hired to mercy-kill a comatose Bran. Their mother, Catelyn, is able to grab the dagger, and it has journeyed through several characters over the course of the series. Eventually, it ends up with Petyr Baelish, who presents it to Bran upon meeting him again as the Starks return to Winterfell. Bran, for his part, hands the dagger over to his sister Arya; it's unclear if he does it because, as the Three-Eyed Raven, he has no sentimental attachment anymore, or because he's seen her use it in one of his prophetic visions.
Along with dragonglass, Valryian steel is one of the few substances that can actually kill a White Walker. Both substances are incredibly rare and nearly impossible to come by, so it has taken a series of incredibly well-connected instances to get the weapon into the hands of someone who has the opportunity and ability to wield it. Ironically enough, it's possible that the weapon's earlier origins actually come from one of Thrones's early villains. Although it's not specified on the TV series, the A Song of Ice and Fire novels reveal, through Jaime and Tyrion piecing information together, that the dagger was given to the would-be assassin by Joffrey Baratheon, who stole it from his "father," King Robert. The dagger has had a long journey to get to this point, but it's turned out to be the weapon that saved all of Westeros.