The lore went that every memory, every incident, every second of your life could be accessed via your blood. If someone was so inclined, the lore continued, a person could access any one of your memories, even those you might have forgotten, simply by drinking your blood. They would know the most intimate details of your life. Every insignificant moment would be clear to them.
As such, a single human would not be able to contain such a vast amount of information. Their brains would simply go into meltdown. That explains why cannibals are both rare and completely insane when they do occur in humans. That is also why cooked meat is the norm. There's less chance of getting an animal's memory from its flesh, but cooking the meat means there is zero chance whatsoever.
However, unknown to the humans (a not-uncommon state of affairs) there lies a world of creatures that are more than capable with dealing with all this information. And these creatures also possess the intelligence to use the information.
Vampires, shapeshifters, weres, demons, angels...more or less every legend that humans dismiss as myth or explain away stalk them. Their apparent dominance over the world is a facade, as they are prey to many of these creatures. Dinner with entertainment. A vampire could drain a full grown man or just a mouthful of blood-the result would be the same. That vampire would possess all of the blood memories contained in that person.
Sometimes, just sometimes, these humans see things that they cannot explain. That make no sense. However, these denizens of myth, they know. They seek the knowledge. They search restlessly for the key to evade..Him. Not God, or the Devil or anything like that. They search to evade the final death, the Reaper, Death himself. These creatures, with their extended lives, grow to love life too much. They take the blood of humans to sustain them, but also in the hope that one of them has the key, buried in their mind.
For it was foretold, that a human has the secret in their blood. That one pure soul was given the key to evading the Reaper, long ago, in a distant life. Now, whatever they once were, they have been reborn into the blind, innocent human population.
Now, the blood will stir. The creatures race for the soul that will set them free. The races compete to claim the prize first-and to destroy it before another learns of the key to defeating Death. Immortality.