Trafalgar D Water Law was sent to The Kingdom, Dragondale, to capture the prince of Dragondale, Monkey D Luffy. Law wasn't told why he had to capture the prince, but he didn't really care. Luffy is a very rootless Prince, some may call him the devil, but only his enemies would be so scared of him, Luffy would torture his enemies in the most painful way possible for fun. But to his people, he was kind and generous. Some of them knew this side of Luffy and still accepted him. Btw, there is haki in this Lawlu.