You must explore Blood Castle and kill its owner: Count Gazini, the powerful vampire mastermind. You should deactivate the altars he built for protection in rooms near his Lair, as they make him virtually invincible.
The altars channel dark energy coming from Gazini’s vampire-minions, who protect and feed them. Stepping on an altar will draw such vampires to you. Kill them to stop the flow of energy, deactivating the altar.