It is 12 BG. Hours ago — or at most a handful of days — Avatar Roku died on his home island.
A volcanic eruption consumed the place where he had lived with Ta Min. Roku woke to trembling earth and ash. He got his wife and the villagers to the boats, then stayed. He used all four elements: an air sphere to buy time, earth trenches to turn lava toward the sea, air again to push poisonous gas, and even the Avatar State to heave magma aside. A second peak blew. Fire Lord Sozin, who could see and feel the eruption from the Capital a hundred miles away, arrived on his blue dragon and fought beside his old friend. Roku saved Sozin when the ground gave way. Then the gas took Roku full in the face. He reached for help. Sozin understood, in that moment, that without the Avatar his plans of conquest could proceed. He left. Fang, Roku's dragon, would not leave. The pyroclastic flow took them together.
That is what happened. It is not what the world has been told.
The official story, in these first days, is that the Avatar died a hero holding back the mountain, and that the Fire Lord arrived as a friend and could not save him. Almost no one stood close enough to see the betrayal. Ta Min watched from a boat. Evacuees on the Refugee Quay speak of ash, of missing neighbors, of a dragon that came home alone. A gate log was quietly 'corrected.' A laundry servant noticed a cloak that smelled of sulfur and not of a long fight. The Crimson Mandate is already collecting witnesses 'for their own protection.'
At the moment of Roku's death, by the cycle of the Avatar, the next Avatar was born among the Air Nomads. No one yet knows which child, or which temple. This campaign does not assume the child is Aang. History is unwritten.
The four nations still stand. Fire Nation colonies already exist in the western Earth Kingdom, including Yu Dao, from Sozin's 37 BG invasion. Roku confronted him then, destroyed the palace throne room in the Avatar State, spared him for the sake of their old friendship, and warned him never to overstep again. That leash is gone.
You have twelve years until Sozin's Comet returns. Peace exists. It will not last unless someone makes it last — or something worse arrives early.