Chronicles of Khaldun: Crux of Eternity
Darklords are figures of such tragic evil that they are drawn into Domains of Dread by some unknown process. They are tied to their domains, and seem to have some measure of control over them. Although darklords cannot leave their own domains, they have some control over who enters and exits.
Count Strahd von Zarovich is probably the most famous darklord known.