Complex sentence: Gemma is a big and muscular man, with black hair and a ironclad arm, since his death his neck has a thick scar as a result of had been decapitated by Jubei.
Split into simpler sentences: Gemma is a big and muscular man, with black hair and a ironclad arm. Since his death his neck has a thick scar as a result of when he was decapitated by Jubei.