He created a powerful a ritual that forever bound his bloodline to the land using the magic of the Waystones, blood sacrifice, and pacts with dark powers. The Haifisch family survived Vlad von Carstein’s rise to power by offering up their beautiful daughters as brides to the new undead lords. Eventually they themselves were eventually given the name of Von Carstein. 
The magical potency of their descendants increases greatly when they are made Vampires, so the two bloodlines have existed alongside each other, intertwined for centuries. When Mannfred von Carstein fell at the Battle of Hel Fenn, they quickly assumed other names and hid themselves, waiting for his return.