The Freljord. Eons ago. In a time where no strife existed, because indeed, there was no mortal tampering to cause it. The Frost Watchers ruled over Freljord, content with their icy domain. But soon they began to lust for more power, beyond the edge of the tundra. They began to lust for all of Valoran to be consumed in their icy rule.
One of the Watchers however, realized that this would inevitably bring strife, and the subsequent war might leave the Freljord tarnished from its original, fantastic glory.
So he created a being to safeguard the true Freljord: One of icy fury, but not of insane bloodlust. However, the power of just one Frost Watcher would be nowhere near enough to protect the life of such a being. So this Watcher created Anivia, a being able to be reborn in the ice.
Surprising the other Watchers, Anivia and her creator successfully stopped the savage conquest that the others had planned. The other Watchers gathered and tried to think of a solution. And the one they came up with proved the extent of their insanity and lack of reason: They would bring mortal life to the Freljord, causing chaos and weakening Anivia and her creator's control.
And so, the Frostguard came into being. But to the horror of their creators', they sided with Anivia and the one Watcher. Infuriated, the Frost Watchers channeled all their power and aimed it at their traitor. He looked them straight in the eye and said he was not afraid to die. He would die with the vision of a perfect Freljord.
The other Watchers, cruel beings that they were, responded to his statement: They did not remove his immortality, but instead drove him to insanity so he would lose the great vision he had. Anivia, upon learning of this, rushed to intervene, but only succeeded in getting herself killed. The Watchers took advantage of the opportunity and tried to end Anivia.
However, in his last act of sanity, the Watcher that had brought about Anivia's existence threw himself at the terrible corrupted ones, saying to Anivia that she was the last hope for Freljord. The corrupted Watchers did succeed in one thing though: Every time Anivia was killed, her memories died with her, so that when she was reborn, and she had no memory of her previous lives. The only thing she awoke with was her purpose: the defense of the Freljord.
However, without her creator to aid her, Anivia began to lose the battle against the Frost Watchers. Just as it seemed Anivia would be unable to stop the greed and lust that was the Watchers, the Frostguard intervened.
The tribe had been waging war against the Watchers for all this time. They were led by Avarosa, and she was supported by her two sisters, Serylda and Lissandra. But they had been unable to make any headway, due to the fact that they had a traitor in their midst: Lissandra. She kept the Watchers informed of all of the Frostguard's actions, and the Watchers repaid her with immortality and witch-like powers. But Lissandra failed the Watchers.
The tribe sent a band made up of their fiercest warriors, wisest tacticians, and most skillful mages, unbeknownst to Lissandra. After a long and brutal battle, the Frostguard's heroes made the ultimate sacrifice: They dragged the Frost Watchers into a bottomless abyss, created by the mages, and contained them within it using their own souls, dooming them to suffer for all of eternity. Lissandra, wrecked with grief, vowed to prepare for the day she could free the Watchers from their ethereal and eternal prison.
Anivia had died in the struggle, but she was simply reborn, with no memory of the battle. And, in time, the other inhabitants of the Freljord followed Anivia's example and began to forget. The sisters tried to maintain the memory, but they failed. And, due in part to the lack of memory of their heritage and history, and in part because of Lissandra's tamperings, the once great and united Forstguard split into three: The Avarosan, the Winter's Claw, and the remnants of the Frostguard. The Frostguard that now existed however was nothing more than a veil of Lissandra's corruption. It existed only to mask Lissandra's true identity and purpose.
The tribes waged almost constant war, which was perfect for Lissandra, but Anivia knew that this was not the vision of the Freljord she was meant to uphold. But, after the great Rune Wars, the League of Legends was established.
With this great system, the Freljord started to gain peace, little by little, despite Lissandra secretly trying to stop it. Anvia was among the first to join the League hoping this wave of peace would continues, and that Freljord would be restored. Unfortunately for Lissandra, the summoners of the League discovered her true identity, and revealed it to all of Valoran.
Lissandra, at that point, was so close to freeing the Watchers she no longer cared about maintaining her secret. The true problem came when the summoners, who had discovered her plan, turned the Watcher's prison into a Field of Justice: The Howling Abyss. This made it near impossible for Lissandra to approach the abyss without attracting attention.
Seeing no other path, Lissandra joined the League on behalf of the Frostguard, hoping to free her masterswhile battling in the Howling Abyss, and help them to bring their vision of the Freljord to fruition: All of Valoran covered in ice.