The Immortal Vampires started killing so much to take the blood. Their cultivations could only improve quickly by drinking enough blood.
Amongst all blood, human blood was the most attractive. It didn't need to be refined. They could absorb it directly and transform it into cultivation and strength. Drinking the blood of savage beasts was also beneficial, but they had to spend a lot of time to refine it. It was relatively more troublesome.
If they could drink human Saint blood every day, an Immortal Vampire's cultivation speed would make a human pride feel despair.
Every time the Immortal Vampires conquered land in the north, they would gather all the human corpses. They'd extract the blood and form a blood pool.
Only the Immortal Vampires that had killed the most enemies in battle qualified to cultivate by the blood pool and improve themselves.