None know where the vampire plague started, but as far as anyone can remember it has always been. Once the vampires were shepherds, overseeing their dominions in the mountains and valleys of Eastern Europe(in exchange for blood and fealty), but when the NecRoman Empire swept across their lands, they were dethroned. Some scattered east, a few south into Christian and Muslim lands, but most went west into the heart of the Undying Empire.
There some of them have sought to reclaim their old lives, maintaining cults that they feed from, while others seek revenge by fighting the officials and rulers of the Empire, assassinating their necromancers and disrupting their rituals.
Ghouls
Ghouls are degenerate vampires. Though vampires can feed on dead flesh, those who do it too many times find their minds atrophying until they are little more than hungry animals. Terrifyingly powerful and intelligent animals, but animals nonetheless.