The Hellsing Hunters are a family of supernatural hunters, specializing in hunting vampires. Though their ancestor, Abraham van Hellsing, is the most famous and well known of the family, their history goes way back, as far back as the dark ages.
The family has a special pact with Alucard and, as long as he doesn't hurt innocence ones, or interferes with their hunts, they will leave him and his coven alone.
The King Slayers are a hidden branch of the Hellsing Hunters, a secret from the rest of the family. They dedicate their time to finding ways to kill a Vampire King. Their end goal is to kill, or at least permanently incapacitate Alucard, and they will destroy anyone or anything that stands in their way.