After 8,000 years, the Wall finally came down in the Season 7 finale of Game of Thrones. Now the Night King and his army of the dead are heading straight for Westeros, and there's really nothing standing in their way any longer. Even so, one has to keep hope alive that the living will be able to win this battle. But how can the Night King be defeated on Game of Thrones? There are a few possibilities.
If one assumes the Night King can be killed just like any other White Walker, then he can be taken care of with fire, dragonglass, and/or Valyrian steel. However, now that the Night King has his own dragon, it'll be incredibly difficult for anyone to get close enough to him to do the deed. Even if Daenerys uses her dragons, Viserion with his blue fire can counteract that attack. Similarly, the Night King is smart and strong. He was able to kill Viserion with one throw of his spear; he's not going to be taken down easily.
Plus, this is also assuming that he really can be killed just like any other White Walker. The Night King is incredibly powerful and has been around for a long time. What if another kind of weapon is necessary to kill him; like the Lightbringer sword, for example.
According to the legend of the Prince That Was Promised, Lightbringer is the sword that was made by Azor Ahai. He had to kill his wife, Nissa Nissa, for the sword to attain its true power, and then wielded the sword to fight "the darkness." Previously, Melisandre believed Stannis Baratheon's sword that he pulled out of the fire created by burning statues of the Seven was Lightbringer, but it's now clear that wasn't true. Therefore, many fans are still theorizing about how the sword could be made and who will bring it to life and wield it to kill the Night King.
No matter what weapon is used to finally off this terrifying villain, it seems fair to assume that once he's dead all the other White Walkers and wights will die along with him. In "Beyond the Wall," when Jon killed the White Walker that was leading a group of wights, all the wights died except for one. This seemed to hint that if you kill the White Walker, all the wights it created will die as well. Therefore if you kill the Night King, the first White Walker, all the White Walkers and their wights should also die as well. Hopefully. But that'll be easier said than done.