I've thought about Moriarty's return for a while. If Moriarty returns, London will burn. London will burn and burn and burn. Ashes to ashes and dust to dust, the world will be set aflame when the king returns. Sherlock thinks he can stop the king. But he can't. Because the world must burn. And Moriarty holds the match. London's demise as told by the third Holmes, Scout Holmes.