Complex sentence: He killed all the assassins and survived all the attempts on his life; however, his mother was killed by an assassin-Like Ryuuki, Seien also had a mother who regretted giving birth to one of the Emperor's children.
Split into simpler sentences: He killed all the assassins and survived all the attempts on his life; however, his mother was killed by an assassin. Like Ryūki, Seien also had a mother who regretted giving birth to one of the Emperor's children.