Frederick van Hal

Frederick van Hal (later known as Vanhal or Vanhel), was both legendary necromancer and progenitor of the Van Hel family of witch hunters.

In 1111 IC, the dead first walked in the Empire. Corpses marked with the black blotches of plague shambled about the land, needing only a will to guide them. They found it in the form of Baron Frederick van Hal. After conquering the land that eventually became Sylvania, he built his keep at Vanhaldenschlosse. During the time of the Black Death, the Empire suffered at the hands of the Skaven, and only the expansion of Vanhal's necromantic domain halted the ratmen's advance. The Skaven and the Undead fought a long and futile war that only ended when Vanhal was assassinated by his apprentice, Lothar von Diehl. The Skaven, however, were left in a weakened state, allowing the future emperor, Mandred Skavenslayer, to drive them from the land. Later generations of the Vanhal line took up the oath of the Witch Hunter in an attempt to atone for the sins of their heretical ancestor.

Source
pg. 41
 * Warhammer Armies: Vampire Counts (8th Edition)