Complex sentence: Egg tells Duncan that Lord Butterwell took no part in the Blackfyre rebellion but one of his sons fought for the red dragon and one for the black that way his house was guarenteed to be on the winning side but both his sons died on the Redgrass field.
Split into simpler sentences: Egg tells Duncan that Lord Butterwell took no part in the Blackfyre rebellion, but one of his sons fought for the Red Dragon and one for the Black. In that way his house was guaranteed to be on the winning side, but both his sons died on the Redgrass field.