They raise the dead using necromancy, which is a magic art the vampires have a near instinctive control over. It stems from death magic, so whenever the wind of death is strong, so is necromancy. Although only more powerful vampires can raise large amounts of the undead, as in units or armies (maybe only allow places with a general to recruit, if possible), the ones that are powerful can reanimate corpses basically at will, and as many as are available in an area. In the warhammer world, any given place is likely to have plenty of skeletons, even zombies, available.

What are these ‘source books’ that everyone keeps mentioning?