"Beyond the Wall" left many Game of Thrones fans shocked and devastated when the Night King killed one of Daenerys' dragons, Viserion. To make matters worse, the Night King then turned Viserion into a wight, or ice dragon, giving him the now recognizable blue eyes of the dead. Now that Viserion is back from beyond the grave, many fans are wondering: can ice dragons be killed on Game of Thrones?
Although Viserion is now technically dead, there has to be a way to kill him for good so he can't be used to help the Night King destroy the world of the living. But it was already incredibly difficult to kill the dragons when they were living, so one can expect it to be even harder to do now that Viserion is under the Night King's control and protection.
It's highly possible you can kill an ice dragon the same way you kill wights and Night Walkers: using dragonglass, Valyrian steel, or fire. However, there are a few problems with this. For one, similar to the giant spear contraption that was made to kill Daenerys' dragons, it would take a sizable weapon to actually do any real damage. And just like the giant spear didn't stop Drogon, it seems unlikely that a large weapon would be enough to kill an ice dragon.
However, the Night King's ice spear was enough to kill Viserion, so a large spear made of Valyrian steel or dragonglass could possible be enough to put Viserion down for good, but it seems unlikely. Another problem is, it's currently unclear if Viserion will still breathe fire or if he'll breathe ice. If he still breathes fire, I can't imagine fire will be able to kill him.
The best way to kill an ice dragon would probably be to kill the White Walker that created it. Before things went south beyond the wall, Jon killed a White Walker and all the wights that Walker had turned dropped dead as well. This seems to imply that if you kill a White Walker, you also subsequently kill all the wights it created. It has yet to be shown if this still applies to any animals White Walkers created, but it'd make sense that it would.
Therefore, if Jon, or anyone, can just kill the Night King, that should also put an end to Viserion. Of course, while that may seem like a simpler task than trying to kill the dragon itself, it could prove to be just as difficult. Just as Drogon protected Daenerys when Jaime had the bright idea to try and kill her, Viserion may now do the same for the Night King. And the fact is that fans also aren't entirely clear on whether or not Viserion is an actual ice dragon or a dragon wight. (Ice dragons and wights are two different things.) You would think the methods for killing them, though, would be fairly similiar.
Unfortunately, no matter how you look at it, the Night King having a dragon in his possession is bad news for everyone. Hopefully, Jon and Dany will still be able to defeat the Night King, but right now the odds don't look so good.