Let me spell it out a little bit better so you can understand just why there would actually be a device capable of destroying the Wall and heralding the end of the World.
Earthquake #1 was the first WMD used by the Children.
Arm of Dorne - A Wiki of Ice and Fire
Around 12,000 years before the War of Conquest, the First Men came to Westeros from Essos by crossing the Arm of Dorne. When the First Men and the children of the forest first went to war, the old songs say greenseers of the children used dark magics to make the seas rise and sweep away the land, shattering the Arm, in a futile attempt to end the invasion of the First Men. Although the shattering of the Arm was successful, it was too late, for the First Men in Westeros had already crossed and the wars went on until the Pact.[1]
Earthquake #2 referencing another post.
This caused the Southern North to turn into marshy swamp that it remains today. Where Howland Reed, Meera, and Jojen are from.
Referring again to the well-known timelines, the hammer was brought down on the Neck during the invasion of the First Men. After the sundering of the Arm of Dorne failed, the children brought the hammer down in a second attempt to thwart the advances of the First Men.
Then the pact was made between men and the CotF. Joramun, who owned the horn, lived during the Age of Heroes in which the Night's King was defeated. The horn is real and has existed prior to the Long Winter when the only magic existed through the Old Gods and Children.
The Horn of Winter, also known as the Horn of Joramun, is a legendary horn with magical properties. It was supposedly blown by Joramun, a wildling King-Beyond-the-Wall.When he blew the horn, he "woke the giants from the earth."[1] It is currently claimed that blowing the Horn will destroy the Wall. [2] Mance Rayder claims to have found the Horn of Joramun in a grave beneath a glacier, high up in the Frostfangs. [3]
When he blew the horn, he "woke the giants from the earth."
Woke Giants? Seriously. Giants are a sentient species that breed just like everything else in the world, and now their extinct. If the Bible or some other ancient text were to use the term 'Waking Giants' you would think of the World shaking. An earthquake.
The giant bellowed again, a sound that shook the leaves in the trees, and slammed his maul against the ground. The shaft of it was six feet of gnarled oak, the head a stone as big as a loaf of bread. The impact made the ground shake.
GRRM is associating giants with making the ground shake, something we know the CotF were able to do.
Why else would someone make a device that could bring down the wall? It would serve no purpose except to bring about Armageddon, yet it was still made and still exists but for a different purpose. The horn Sam found was clearly made without metal, something the CotF would have created. It was deliberately broken to prevent it being used, for if it hadn't been, Sam would've brought down the wall when they tried to blow it after finding it.
Those of you saying that Bran will defeat the Wall's magic. D&D will diverge from the books, but holy fuck that's some lazy fucking writing. GRRM doesn't write like that, and every major event has clues littered in previous books.
The Horn of Winter wasn't built to destroy the wall, it was built to destroy men. Sam has it in Oldtown.