In the Harry Potter Series Harry, his 2 best friends Hermoine and Ron, along with Harry's nemesis Malfoy, have all been given Detention. They are to go with Professor Hagid, who teaches 'Care of Magical Creatures', into the Forbidden Forest to help track an injured Unicorn. Harry ends up seeing a hooded figure bending over a slaughtered unicorn, drinking its blood. Later Firenze the Centaur tells him:

"...it is a monstrous thing to slay a unicorn..Only one who has nothing to lose, and everything to gain, would commit such a crime. The blood of a unicorn will keep you alive, even if you are an inch from death, but at a terrible price. You have slain something pure and defenseless to save yourself and you will have but a half-life, a cursed life, from the moment the blood touches your lips." - Harry Potter and the Philosopher's Stone, by J.K. Rowling.

Voldemort, the villain in the Harry Potter series, is acting like a Vampire, feeding on unicorns. The vampire's of fantasy horror stories, are animated dead, essentially undead. Blood is life and they violently, forcibly drain blood from their victims, drinking the blood of others to sustain their existence. Where did the idea that blood can give life to the dead come from?

The truth is this idea about the power of blood came from the Bible. Jews, Muslims, the early Christian Church and Vampires all share a common belief, that blood is in some way, life itself, able to give life to another, and to conquer death. In fact the only way to defeat death is to use it's opposite Life - and according to the Bible, Life is in the Blood.

After Cain killed Abel the Bible says Abel's blood cried out to God from the ground. King David was not allowed to build The Temple, even though he was a man after God's own heart, why? Because there was 'too much blood on his hands'. When God created a way to achieve atonement, to put things right while satisfying Karma. It had to involve blood, because Sin = Death and only Life (blood) can defeat Death. The concept may seem very strange to us, but it is the backstory to Jesus sacrifice for atonement and something that every apostle (who were all Jewish) understood well.

Life is a gift from God and is sacred. We are not allowed to take life violently from another person. For the Jews, and many others, even animal life is sacred. Jews believe animals are only to be killed for food or the sacrifice to save your soul, and even then the blood must be drained and buried out of respect for its holiness. So blood is sacred but I would still encourage you to give blood to a local blood bank if you are able. Giving it does not endanger your life and may indeed grant life to another.