Breaching of the Wall

The Breaching of the Wall is the event that marks the closing of the Conflict Beyond the Wall and the beginning of the dreaded Great War. After amassing a massive Army of the Dead beyond the Wall for over two decades, the Night King leads the White Walkers and the wight army to the Wall. There, the Night King, mounting the ice dragon Viserion, attacks a portion of the Wall at Eastwatch-by-the-Sea, melting it down and creating a breach allowing the White Walkers and the wights to cross into Westeros and spread their magic, marking the return of the Long Night in the world.

After the War for the Dawn thousands of years ago when the Long Night and the White Walkers first descended upon Westeros in the Age of Heroes, the First Men and the Children of the Forest, with the aid of giants, raised the Wall under the eye of Brandon the Builder to prevent the White Walkers from returning again. While the First Men and the giants physically erected the Wall, magic had been carved into its ancient foundations by the Children to prevent the dead from crossing. The building of the Wall did leave some of the First Men trapped north of it; these people became the Free Folk. The ancient order of the Night's Watch has stood as the watchers on the Wall ever since.

Prelude
When the First Men crossed the Arm of Dorne from Essos to Westeros in the Dawn Age, they came into conflict with the native peoples of the continent, the magical Children of the Forest. Centuries of warfare passed with the First Men cutting down the Children's sacred weirwood trees and forests while the Children wielded magic and dragonglass to combat the First Men, including destroying the Arm of Dorne and flooding the Neck. Secretly, Leaf led a pack of Children in turning mankind's greatest weapon against them: man themselves. After undergoing a ritualistic process, Leaf shoved a dagger of dragonglass into the chest of a captured First Man, creating the Night King. Peace was eventually settled between the First Men and the Children of the Forest, the former of which even adopted the Old Gods of the Forest worshiped by the Children.

Nonetheless, 2,000 years later in the Age of Heroes, the Night King led the White Walkers onto Westeros for the first time from the far northern Lands of Always Winter, bringing with them the Long Night and slaughtering all that lived in their path, raising the dead as wights. The First Men and the Children eventually came together to fight back against the seemingly unbeatable White Walkers in the War for the Dawn and miraculously drove them back to the far north. Brandon the Builder oversaw the raising of the Wall to prevent the possible return of the White Walkers, and the Night's Watch gathered for the first time to man the Wall while the others trapped beyond the Wall became the Free Folk.

Over the centuries, magic, the White Walkers, the Children of the Forest, and giants had faded into legend and were seen as nothing more than fairy tales. The primary opponents of the Night's Watch became the Free Folk, or the wildlings, who sought to cross the Wall and inhabit the North. Nonetheless, around the time of Robert's Rebellion 8,000 years after the first Long Night, the Night King and the White Walkers had reawakened and risen from the ice. Massacring the Free Folk beyond the Wall, the wildling chieftains united behind Mance Rayder, the King-Beyond-the-Wall, to lead them to safety. Around the same time, Jeor Mormont, the 997th Lord Commander of the Night's Watch, led the Great Ranging beyond the Wall to confirm reports of the wildling tribes coming together and reports from their own rangers that the White Walkers had indeed risen again, especially after Jeor Mormont and Cotter Pyke themselves had personally been targeted and attacked by wights.

The Great Ranging confirmed the reports, but few in the Seven Kingdoms paid any mind to it, believing it to be false and preoccupied with their own civil war. Jeor Mormont died in a mutiny after the rangers had been attacked at the Battle of the Fist of the First Men. The wildling armies under Mance Rayder later attacked the Wall but were soundly defeated. Jon Snow, who had been Jeor Mormont's own steward, became the 998th Lord Commander and made the controversial decision to allow the wildlings to cross the Wall to save their lives and prevent them from becoming wights. Though Jon himself store down the Night King at the Massacre at Hardhome, he was ultimately killed in a mutiny led by Ser Alliser Thorne for his decision in allowing the wildlings to cross the Wall. However, Jon was subsequently resurrected by the red priestess Melisandre at the behest of Ser Davos Seaworth. The mutineers were hanged for treason, and command was passed off to Eddison Tollett as Jon was relieved of his vows.

Jon, a bastard of House Stark and nephew to the late First Ranger Benjen Stark, joined his half-sister Sansa Stark in liberating the North from House Bolton at the Battle of the Bastards, seeking to restore Stark rule over the North from Winterfell and unite the Northmen against the marching Army of the Dead. Jon was proclaimed the King in the North by his new bannermen. Nonetheless, when Daenerys Targaryen begun her invasion of Westeros, Jon traveled to Dragonstone to treat with the last Targaryen. Daenerys had three fire breathing dragons, the first born in over a century, and controlled the reserves of dragonglass beneath Dragonstone which could be used to combat the Army of the Dead and the White Walkers. Initially sharing a tense relationship, the two grew to trust each other.

Nonetheless, her Hand, Lord Tyrion Lannister, who had previously met Jon before, counseled that Queen Cersei I Lannister and the high lords of Westeros would need to see the Army of the Dead as Jon had before if Westeros was to unite against the White Walkers. Agreeing, Jon led a group of warriors and wildlings beyond the Wall to capture a wight at the Wight Hunt, including Ser Jorah Mormont of Daenerys's Queensguard and a fellow Northman, the bastard son of Robert Baratheon Gendry, the wildling Tormund, and Brotherhood fighters Lord Beric Dondarrion, Thoros, and Sandor Clegane as well as several wildlings. Though the group successfully captured a wight, they were ambushed by a large contingent of the Army of the Dead led by the Night King and his lieutenants themselves. Gendry ran back to Eastwatch-by-the-Sea to get Davos to send a raven to Dragonstone asking for Daenerys's help. As Jon and his fellowship were attacked, Daenerys arrived with her three dragons: her mount Drogon and his brothers Rhaegal and Viserion. While Jon fought off advancing wights to allow the survivors of his group to mount Drogon with Daenerys, the Night King launched an ice spear at Viserion, killing the dragon. Daenerys and the others were able to escape, however, while Jon later returned to Eastwatch after being saved by his long lost uncle Benjen, who sacrificed himself for Jon.

While Jon, Daenerys, and their entourage traveled to attend the Parley in King's Landing, the Night King reanimated the corpse of Viserion as a wight and turned the dragon into an ice dragon.

The breaching
Tormund and Beric Dondarrion talk about the Wall, with Tormund noting how he still hasn't gotten used to how high it is. They then see the massive wight army of the dead below marching out of the Haunted Forest, led by the White Walkers riding their wight horses. Initially terrified, they are relieved to see them stop as the Wall intended. Suddenly, however, Viserion, now an ice dragon and mounted by the Night King himself, flies by and unleashes blue fire onto the Wall. The Night King continues to have Viserion breathe his magical blue fire onto the Wall, melting and destroying it where Eastwatch-by-the-Sea stands. The brothers of the Night's Watch and the wildlings flee as Tormund screams for them to run, many falling off as the Wall crumbles. Tormund and Beric freeze in horror, watching the Night King using Viserion to destroy the Wall. A breach is created, allowing the White Walkers and the massive wight army of the dead to cross into Westeros, marking the return of the Long Night once more.

Aftermath
The White Walkers and the wight army of the dead marches onto southern Westeros through the breach at the Wall as the Night King flies above them, mounted on the ice dragon Viserion.

In the books
In the A Song of Ice and Fire novels by George R.R. Martin, such an event has not yet occurred. The story of the Night's Watch last ended at the Mutiny at Castle Black in A Dance With Dragons, and thus the Wall still stands. It is possible - and likely - that the assault on the Wall by the Others and the wights will occur in one of the forthcoming novels, The Winds of Winter and/or A Dream of Spring.