What secret does the key of destiny hold?

Why do these ancient Western vampires come to grab it? What the hell are they doing?

The evil spirit slowly lowered the dragon sword in his hand, then looked at Miss Mary in front of him and said, "Tell me, why do you vampires want the key of fate?"

Miss Mary shook her head decisively and said, "As I said, it's the secret of our blood."

When the evil spirit heard what Mary was saying, he said coldly, "Well, since you don't want to say it, I will never give you the key of fate."

After the evil spirit had said so...