Two main reasons people come to magick is either:
(1) They want to become a vampire
(2) They want to be a mermaid.
These are quiet impossible and i would like to shed some light on the subject's.
Vampires:
The First Vampire Story was Dracula, although some beliefs stretch before 1897 this was the first published works. Now At this time people believed them as complete myth (Like pokemon and unicorns) But then (for some reason) people started digging up graves and noticed hair and finger nail's had grown, this led them to the belief that people where living after death. (No we know that this was caused by moister leaving the body caused the skin to tighten and give the illusion of hair and nail growth.
Now people got more aggressive, people with darker trates and appearances were locked up and tested on (of coarse these were ancient times and these "Test's were usually some form of torture).
Of coarse the myth was stretched into catholic beliefs and That's where The theories of Holy water and cross's would kill such a being came from.
Vampires were creatures that drunk blood to survive said in a story, not anything to believe in, just an old book and awful common sense, not a thing you can become.
A disorder of the mind called vmhd (Vampirism mental health disorder) is a disorder of the mind (usually a extreme case of paranoia) Where someone either believes:
1) Someone is stealing there blood and they need to drink others to survive.
or 2) Something is living inside drinking there blood and they need somebody else's.
This disorder is quiet uncommon and generally discovered early one, before someone is hurt.
The term vampire is often used to refer to someone who takes something from someone in order to survive, so naturally psy-vampire is a term for someone who takes (often subconsciously) psy-energy from one in order to survive.
Please stop asking to become a vampire, it was a story that got out of hand, ignore people who claim to be a vampire and spells that say that they will make you one, it will not work.
Mermaids:
The first known mermaid stories appeared in Assyria, ca. 1000 BC. The goddess Atargatis, mother of Assyrian queen Semiramis, loved a mortal shepherd and unintentionally killed him. Ashamed, she jumped into a lake to take the form of a fish, but the waters would not conceal her divine beauty. Thereafter, she took the form of a mermaid?human above the waist, fish below?though the earliest representations of Atargatis showed her as a fish with a human head and legs, similar to the Babylonian Ea. The Greeks recognized Atargatis under the name Derketo. Prior to 546 BC, the Milesian philosopher Anaximander proposed that mankind had sprung from an aquatic species of animal. He thought that humans, with their extended infancy, could not have survived otherwise.
There have been other stories, my favorite is an old irish one (being irish i would know) It's called the seven stones of Sligo
The story goes: A Prince was struggling to find love before he became a king so he looked out to sea to try find his answer.
One day he saw a beautiful woman on the coast of sligo, singing and brushing her hair. Every day he went back to watch her. One day she sung a song that he could not understand the words of, but it was an alluring song so he decided to go closer to her, he went to her and she was frightened, she tried to run but she looked very unversed with her legs, and could only walk, so he asked her, why she sung everyday, She replied "I sing to the the ocean". A bit of water then hit her legs and she turned into a mermaid, she explained who and what she was and how lonely she was, he then proposed on the spot and she never Touched water until 10 years later. 10 years later the king brought his youngest son (3) to a far away forest, it started pouring rain, and lighting struck a nearby tree and under it was a long scale.
He knew that his wife would have to return to the water so he went to tell her. She said "If i don't return to the ocean i will die, i must go back" So she brought all her 7 children to the shore of Sligo, one by one she kissed them, one by one the each turned to stone, she then returned to the water, and her husband died of a broken heart.
Sorry that's my favorite story ever :D anyway being a mermaid is impossible by mere Nature alone, and anyone who claims to be a mermaid, ignore them, A spell that says it will turn you into a mermaid, ignore it.
(sorry i had more on vampires, there was just more sources :D
Sources:
Wikipedia and my own collective knowledge :D