The Vampire Kings are a group of five incredibly powerful vampires who rule the Shadowfel equivalent of The Middle Kingdom. Their power is derived from spirit shackles, which bind a spirit in place and force it to bestow powers upon the shackle's creator or another being of their choosing.
In time immemorial, four Vampires gained the ability to shackle spirits. These four (Vladimir, Swayne, Hastings and Lockhart) became the Vampire Kings and used their power to wipe out all other Vampires and then divide the continent among them - Vladimir in the North, Swayne in the West, Hastings in the South and Lockhart in the East. They then began warring amongst themselves for shackles, naturally.
This equilibrium was shattered when Swayne made his infamous deal with Balvan the Millstone. In exchange for Balvan's eternal servitude, Swayne turned his daughter Alice into the Vampire Alyssa, thus saving her from the terminal case of soul mismatch from which she was suffering. However, Swayne made the mistake of teaching Alyssa the secret of spirit shackling and she eventually betrayed him, taking a sizeable portion of his territory for herself and leaving Swayne with the smallest territory of any of the Kings.
This new five-King system continued until Rygax launched his successful rebellion against Vladimir. Vladimir was struck down and powerful Spirits held off the forces of the other four Kings long enough for Vladimir's shackled spirits to escape. However, unbeknownst to anybody else, Vladimir had turned his lover, the half-Giant Cornello, and taught him the secret of spirit shackling. Cornello rose as the new King of the North, shackling Vladimir's spirits before they could flee and rendering Rygax's actions ultimately fruitless.