In the first episode of Game of Thrones Season 7, Euron Greyjoy disembarked to King's Landing in the hope of making an alliance with Cersei. To try to convince her, he promised that he would acquire some kind of priceless gift for her that would prove just how serious he was. Some have speculated that the gift might be the Horn of Winter. But wait — what is the Horn of Winter on Game of Thrones? Let's just say that it serves a very dangerous purpose.
Also known as the Horn of Joramun, the Horn of Winter is a mystical horn that's mentioned in the A Song of Ice and Fire series. It belonged to Joramun, the King Beyond the Wall, who used it to wake giants from the earth. It's also been speculated in the communities of the Free Folk that the horn can bring down the Wall, which is the only thing separating Westeros from the hordes of undead gathering an army way up north. It's a powerful item, but it's unlikely that Euron is in possession of it, because the magical horn he says he owns in the books is an entirely different one.
Euron insists that he has a horn called Dragonbinder that can control dragons, which would give him the upper hand in a world where dragons are once again flying through the skies. It's possible, of course, that both coincidentally special horns are one and the same, but there's not really enough evidence to say so conclusively. Either way, both horns are highly sought after, with many searching for them or claiming to have one.
But the Horn of Winter could be an important asset in the upcoming battle against the White Walkers. While its ability to bring down the Wall may make it seem like it's only here to benefit the army of the dead, it could actually be instrumental in defeating them. It was said that Joramun was integral in defeating the Night King many, many thousands of years ago — which he could have done with the help of his handy horn.
Mance Rayder claimed to have found the Horn of Joramun, but it didn't seem like he actually had the real one because it was burned along with him. Like Euron's assertion that he owned Dragonbinder, Mance's declaration could be all smoke and no fire. It doesn't seem like the Horn of Joramun has truly been found at all, meaning its eventual discovery could be a real game-changer. It could be a destructive force that does more harm than good, or it could have unseen advantages.