Faced with extreme terror, a person is incapable of making any rational response.
Arthur Dayne, the Sword of the Morning, knew things were going badly the moment that brave fellow drew his bow, but he could not save him, terrified that he would not even be able to save his own skin.
This dragon was simply too massive. It was not an opponent any individual could conquer. Standing nearly one meter ninety tall, Arthur Dayne was not even enough for a single bite.
Chapter 124