Long ago, the three realms—the Realm of Flesh, the Realm of Spirit, and Half World—lived together in harmony.
Then, everything changed when the Fire Nation attacked.
Somehow, the realms were severed, leaving Half Life—the realm in which those departed from the Realm of Flesh were condemned to relive their greatest trauma over and over and over until they resolved their mortal tribulations and entered the Realm of Spirit—to devolve into despair, chaos, and perpetual pain. Only when a child is born in Half Life—an impossible thing in this realm of repetition and patterns—can the Realms be brought back into harmony. Or so the prophecy says.