After a long wait and a seven-week hype, Game of Thrones Season 7 certainly ended with a bang, as the most anticipated moment in the show's history proved that blue is indeed the warmest color. As the Night King used a recently reanimated Viserion to breach the Wall, it was impossible not to wonder how exactly he managed to break the magic that sustained it. Perhaps the Night King took a page from Jon Snow's book and decided to gather all his strengths — the giants, wights, White Walkers, the stormy winds, Viserion and himself — to get the job done.
But whereas Jon had six companions in his motley crew for his face-off with the army of the dead in "Beyond the Wall," the Night King seems to fall one group member short; unless, he's already in possession of a certain horn fabled to contain magic powers able to break the Wall. Add to that Game of Thrones' seven seasons of magic revival, and it's clear the Night King couldn't have picked a better moment to cross south.
The Night King's Magnificent Seven
Far from being dubbed the Suicide Squad (unlike Jon's gang), the Night King's ensemble — the White Walkers, wights, giants, Viserion, the blizzard, the horn that brings the winter, and the Night King himself — is definitely more likely to pose a threat than to be in any danger right now. If before, with his growing army of dead wights and his Craster-factory of #WhiteWalkers, the King was already a force to be reckoned with (Hardhome, anyone?), with giants and an absolute-zero spitting dragon it's hard to imagine what the Night King can't do.
Created by the Children of the Forest to protect them from the First Men, the Night King became the first ever White Walker. He then proceeded to create more White Walkers, who are turned when they're alive and, presumably, at a young age (as was shown when the Night King turned one of Craster's sons). White Walkers are impervious to fire, drastically stronger than men and, as confirmed in "Beyond the Wall," they have the power to reanimate the dead such as the Night King.
Where White Walkers are the generals of the #NightKing's army, the wights are his foot soldiers. They too are significantly stronger than humans, though they lack any other magical abilities; they're always turned after death and, unlike the White Walkers, they seem to decompose at a much faster rate (even the giants). As Viserion's eye turned blue at the end of "Beyond the Wall," there was some speculation on whether he'd come back as a wight or as a White Walker (since the Night King touched his nose to turn him). Sadly though, it appears Dany's sweetest child has turned into a wight, one that is twice as powerful as before and who spits blue fire/absolute-zero ice beams/magic flames — or whatever you wanna call it.
The Night King's final weapon might already be the Horn of Winter, which would definitely explain how he could "wake giants from the earth." Yes, I know, giants are their own thing and they didn't do much in the tearing down of the Wall, but Viserion is a giant in his own right — who brought the Wall down all by himself.
Magically Built, Magically Destroyed
Built some eight thousand years ago, the Wall is believed to have been constructed using both mundane tools and magic to defend Westeros from the White Walkers. However, thousands of years went by without an invasion and the Wall's purpose changed from protecting the realms from an invasion of the dead to keeping the windlings north. It's huge, long and (somewhat) kept by the Night's Watch, so how did it succumb to Viserion's attack? Even if it were physically broken, the magic should hold it in place, right?
The problem with magic in Game of Thrones is that people hardly believe in it anymore. Back in Season 1, Maester Luwin assured Bran that "magic was once a mighty force in the world, but no longer." Too bad the maester of Winterfell didn't survive to see dragons flying over the Wall, because surely if dragons exist once more, then so does magic. And if it exists, it most definitely can be broken.
There are three key factors that could easily justify the Wall losing its magical defenses and tumbling down, and the newly-appointed Three-Eyed Raven can be held accountable for all three of them. First, Bran had a vision of the Night King without the Raven's guidance, which led to his being touched and branded by the King. This, in turn, allowed the King to enter the Three-Eyed Raven's cave and kill not only the Raven, but the last of the Children of the Forest. Mind you, it was the Children's magic which helped build and magically secure the Wall; hence, no Children left means no magic left in the Wall as well.
Finally, it was Bran's vision of the Night King marching to Eastwatch-by-the-Sea that led Jon and his men to venture beyond the Wall to capture a wight, get themselves into an impossible situation, and cause Daenerys to fly her dragons to help out. Game of Thrones theorists, who think Bran is in fact the Night King himself, might actually be on to something since without Viserion it'd be much harder to bring down the Wall — magic or no magic.
The Saving Grace
In the Game of Thrones Season 7 finale, Daenerys shared with Jon the tragic story of the extinction of dragons, how they went from savage beasts used to conquer the Seven Kingdoms to serving as entertainment in the Dragonpit. Having to adjust to a more civilized world, Dany's ancestors began chaining and restricting their dragons, causing them to grow less and less, until the last dragon was no bigger than a dog.
Clearly, it won't take centuries of being enslaved by the Night King for Viserion to dwindle and die. But, as Daenerys told Jon, "a dragon is not a slave," so there's the slight chance that Viserion's powers will fade faster than they normally would because he's being mind-controlled by the Night King. And, assuming that Bran isn't the villain of Game of Thrones in the end, he could eventually hone his powers enough to warg in Viserion. Since the dragon is now a wight, he's not a sentient as he once was, which would allow the warging; thus, Bran would take control of Viserion from the Night King and become a rider as well.
There's at least one year before we find out whether or not the protective magic of the Children of the Forest is truly gone for good, or if the Night King will eventually lose control over its new pet dragon. But, if Game of Thrones' final season turns out to have the same fast pace as this last one, it's unlikely there'd be time to spend discussing how exactly the Wall was broken and its magic lost. After all, the army of the dead is marching south and winter has most definitely come.