Millenia ago, the only white walker known in existence, the one who would later come to be called the Cold-Hearted, reigned terror in Westeros and was set on a course to destroy all humans and Children of the Forest on the continent.
Four heroes of Westeros, with their dwindling armies, rose up to drive him back using artifacts forged from the gods themselves. These heroes began by building The Wall using the combined forces of the three artifacts at their disposal – Earth, Water, and Fire – in addition to one not under their command – the constant Air magic being forced south from the Cold-Hearted’s homeland.
This proved not to be enough to contain such evil.
The Wall was staffed by heroes in their own right, though without any godly capabilities. The Night’s Watch had not officially been founded at this point, but these heroes were the first shields that guarded the realms of men.
This, too… was not enough.
The four heroes decided they needed to take the head off the beast. After an extremely long and harrowing journey, this group of heroes managed to track the Cold-Hearted to the far north. They’d managed to corner him by a lone Weirwood. In an incredible battle, the four heroes prevailed, though they could not actually destroy him. It was as if he, himself, was a godly manifestation.
They managed to weaken him significantly, and cleverly used the Weirwood tree as a prison, which would also allow the Children to keep an eye on the Cold-Hearted from a great distance. They’d used the same formula they’d used to create The Wall to seal this force of evil forever. Except this time, they had secured the Cold-Hearted’s artifact, making this seal even stronger than The Wall. Combining these forces together so closely had an unintended effect: it made each of them very sensitive to each other’s presence when activated.
Afterwards, they made their way south, expecting this would finally be the end of his influence.
Was this finally enough?
The four heroes each kept one artifact. The Children would retain Earth; the Valyrians would retain Fire; the Rhoynar would retain Water; and the First Men would retain Air.
Once every moon cycle, the Children would remote view the Frozen Weirwood proving they’d been successful. Until one day, they were blocked from viewing him ever again. No one knew the true reason for losing this ability, but the Westerosi immediately stepped up their efforts as Keepers of The Wall… the Night’s Watch as it came to be known.
Without the ability to remote view the Frozen Weirwood, the events became lore… and then… old wives’ tales to scare children, in line with stories of grumpkins and snarks beyond The Wall. After a few generations, any sense of cohesiveness and cooperation was long gone, leading humans to battle over territory and power.
All of the factions tied to the original heroes retained their artifacts, keeping them safe by allowing only those worthy of the cause to carry them. Each faction had their own criteria for someone to become an artifact holder, but generally the current holder managed the responsibility of keeping the item safe as well as deciding who would be worthy to carry it going forward.
Going back to when the Cold-Hearted was initially imprisoned, he was weak and his time seemed to have come to a close. He was physically bound and unable to break the seal.
However, with time, he became one with the Weirwood and learned to use its sprawling roots to corrupt the land around his prison. His power continued to grow, eventually splitting the Weirwood from the Children and preventing their surveillance.
After many years, this growing power enabled him to draw in new people with their own innate powers, corrupting them to the core. He was now beginning to gain followers. And with more followers comes more power. Over time, the corrupted followers built a giant Ice Necropolis with this Frozen Weirwood throne-of-sorts at its heart, amplifying his reach.
Powerful mages developed an Antimagic Field around the central chamber to protect the Cold-Hearted, then sealed the chamber with shards that they would keep with them at all times. These mages called themselves the Frostbound Vanguard.
The members of the Frostbound Vanguard would change over time, as sometimes the members were living, and sometimes… undead. But they were always in control of the ice shard keys.
Eventually, another group of heroes, untouched by this corruption, began a most improbable journey. Not only had they defeated every monster and trap in their way, built an outstanding alliance, and overcome every barrier thrown at them, they had become artifact holders themselves.
Over their journey, they’d come across four corrupted followers of the Cold-Hearted that were especially powerful, the latest Frostbound Vanguard.
This Frostbound Vanguard made it their mission to recover the artifacts to unlock the Weirwood prison to allow themselves to ride alongside the greatest force the world would ever see.
Initially, the Vanguard attempted to recover the artifacts from the Unlikely Assembly through various means: by force, by cunning, by corrupting those around them… but when it was clear these heroes would not be stopped, they regrouped at the Ice Necropolis and prepared for their arrival to take what was theirs.
Now, after many human generations have come and gone since taking your stations, you are finally rebuilding the army of undead that your former selves decimated.
Seven hundred years pass before a giant explosion destroys the Valyrian peninsula, killing many in the process.
This explosion was no random event.
The remains of the Cold-Hearted had been melting through the world, like thermite through metal, ever since you’d encased its remains in a new prison. Eventually, its essence reached the other side of the world and collided with the strength of R’hllor in the volcanoes of Valyria.
Finally, the god that once had the strongest following is now weakened, and is rapidly losing followers, further contributing to its demise.
Building their army is becoming easier.
They are gaining followers in the world beyond The Wall.
The other gods are weakening.
Air will have its chance once again.
All they need… is time…