Gaining the Shadow ability
Presumably, life couldn't have been any better for the life of Arthur and his people. Not only had they vampires helped him in great battles, but the technology that they didn't have access to was taught to their people and spread around. Medicine and knowledge were passed that saved hundreds and thousands of lives. The kingdom name fame and fortune was spreading around because of these things.
The things that had happened he could have never dreamed of, but there were problems in all this. The first being that Arthur seemed to never age. Most vampires could control their ageing process and slow down, but as for those that were deemed the first vampires, including himself. It was as if they were stuck. Never growing older or younger. Soon others around would start to notice. While some thought he was blessed by gods, others did believe that it could have been the work of demons.