How

My grandmother used to tell me stories about the old days, a time of peace when the Avatar kept balance between the world. But that all changed when the Fire Nation attacked. Only the Avatar mastered all four elements. Only he could stop the ruthless firebenders. But when the world needed him most, he vanished.

Twice...

We failed. We failed the world when sozins comet passed and Phoenix King Ozai won...

And the world hasn't been the same since.