Complex sentence: While his power is active, Crossbones does not appear to be entirely impervious to the flames he generates; after the flames died following his first usage, his mask appeared to have been burnt away and his face was heavily scarred.
Split into simpler sentences: While his power is active, Crossbones does not appear to be entirely impervious to the flames he generates. After the flames died following his first usage, his mask appeared to have been burnt away and his face was heavily scarred.