Genesis
The Four Nations are no more.
Long ago, there were only four types of Bender, and each had their own nation, a harmony of four balanced and maintained by the Avatar.
Then, change. The Avatar was unseated, and war and conquest ruined the old Four Nations and begat a Fifth, wherein the Benders intermingled and intertwined inextricably. Therein sat the Avatar Resurgent, striking down those who wished to upset the balance, whatever their origin and whatever their nominal goals.
Inevitably, the old Nations fell to the new, as the Diatessaron proved irresistible. All types of Bending flourished.
Then, change. A new type of Bender appeared, among the non-Bending inhabitants of a small and isolated island that had been inconsequential for centuries.
The new Benders worked their will not on Earth, Air, Fire or Water, but upon Life Itself; and thus was the Great Tragedy prefigured in their midst.
Most Life-Benders were Plant-Benders, but a very, very few could Bend Flesh. Flesh-Benders were feared from the beginning, even amongst their own, for the horrors that their Bending inevitably wrought. Periodically, the Flesh-Benders were purged, sometimes by their own hands, but they always re-emerged. So it was that the Life-Benders were bound to their Island, lest a Flesh-Bender bring doom upon the world.
The quarantine failed.
Ninety years ago, the Avatar vanished, and the true Horror of the Flesh-Benders was revealed. A small boat landed on the shores of what had been the Earth Nation, bringing with it the Reaving, a plague that Bent those few victims it was not merciful enough to kill into monsters filled with an insatiable bloodlust.
Within a month, the Reaving had spread to every corner of the world--save only the Island of the Life-Benders, whose ostracism proved their salvation. Their land flourished, their Warders turning back even the most determined efforts by bearers of the Reaving to spread that horror to their land--and the Flesh-Benders were purged (again).
While the Reaving killed off most of the nations, some few in each land proved to be immune; the Reaving-Bent Horrors slew most of them in turn.
Within a year, Mankind had nearly been wiped away.
On the Reaving-ravaged lands, four pitiful remnant nations arose, fighting back the Horrors day by day, ever perched on the brink of oblivion. These were in no way the Four Nations of old; within each, the Watch of the Diatessaron was maintained, as it proved the only way to hold back the Horrors of the Reaving; and all of them were in the old Water Nation, far to the north.
Three of these nations huddled close to each other on one mass of land; the last was on an island closer to the old Earth Nation lands.
The Nation of Gnorrs was a seafaring brood, braving the waves to hunt the sea-borne Horrors.
The Finn’s-Land was mostly a lake-bound archipelago; there, a different manner of Diatessaron was practiced, as it had been since before the old Four Nations themselves were founded.
Between the Gnorrs and the Finn’s-Land were the Svensk, a proud and aloof bunch who had kept the Watch and Ways of the Diatessaron the most completely of the four. Despite this, though, they rejected the Avatar as being “mere myth”.
The one island nation, the Mark of Denn, had been peopled by refugees from the main lands, and these mounted mighty efforts to reclaim their lost homes, but to no avail, as they had no Benders amongst them at all.
Eventually, the Life-Benders awoke to the fact that the future of Man now rested with them, and the very few other survivors in the new nations. Tentatively, the extended their hand in friendship to the survivors, who accepted with more or less alacrity, according to their nature.
Now, ninety years after the Reaving struck, life has settled back into a familiar rhythm. The Flow of Life governs and rules the new nations, even as the Reaving still strives to disrupt it.
Those not of the Reaving who yet cannot or will not follow the Flow have a different fate: exile.