They Sold Me Out, I Command Demons!

The kingdom he had defended for a hundred years would not keep him. He spent the whole of his cultivation holding the demon army off it, and in that time his mother was pushed off the throne, his brother took it, and the woman he was engaged to went over to them. The people he had been shielding cursed his name. He cuts his ties with the kingdom and leaves it to itself. The court's own decisions open the capital to the demon army, the city burns, and the country goes down with it. He takes the power off the men who did this to him and makes them immortal, then leaves them in the ruins where the pain does not stop and there is no way out of it. After that he goes at the heavenly order that had been draining human beings, breaks its body apart, and puts his weight behind the empress of a neighbouring realm as she rises to godhood.