The crown was used to keep a seal intact, which held several evil beings sealed, with the crown removed the seal broke and everything trapped in that cementary was set lose.
after releasing the vampires, but before becoming the 3 "sisters" thrall there are Zombies in the graveyard that attack you.
They use them in order to keep humans away since they are trapped in the graveyard by their masters curse.
Considering the lack of any food they feed on the corpses in the graveyard, likely turning them into more zombies, once you start bringing them human sacrifices though they get an alternative food source for awhile.
I didn't notice a change in enemies from before becoming a thrall and after the sisters moved to castle Tepes... or what ever remains of the former castle