Black Blood Brothers

Ten years after the Holy War in Hong Kong, Mochizuki Jirou, aka the Silver Blade, and the lone hero who fought and defeated the Kowloon Children despite the loss of his lover, returns to Japan with his young brother, Mochizuki Kotarou. The two quickly discover that the Kowloon Children who survived the Holy War are seeking to infiltrate the "Special Zone" ,a thriving city protected by an invisible barrier that will not allow Kowloon Children entrance, unless they're invited. (myanimelist.net)