Think blood
Think stem cells
Think growth factors
Myths:
Do vampires live for ever?
Blood countess Elizabeth Báthory?
Always a young virgin male/female virgins are pure virgins are without sickness.
A sacrifice for blood, always blood.
Other rituals dealing with blood.
Expand your thinking Q says.
Why do these things ?
Who might benefit? Primitive misunderstanding of modern science. Drinking blood ,stem cells,growth factors will not work but a primitive (relative)people with limited knowledge might think it would or will . Then there is always the adrenochrome .