Complex sentence: The knight responsible for this is Ser Gregor Clegane (Conan Stevens), also known as "the Mountain" due to his towering size, a Lannister bannerman and the Hound Sandor Clegane's brother.
Split into simpler sentences: The knight responsible for this is Ser Gregor Clegane (Conan Stevens), also known as "the Mountain" due to his towering size. He is a Lannister bannerman and the Hound Sandor Clegane's brother.