Vampires! Though not all versions.
Also Lich. But again, not all versions.
EDIT To Elaborate:
What appeals to me about these two is that they are both a sacrifice, a half a life as a more powerful being. They are therefore they are tragic curses that make dark characters not so simple.
Vampires can have many enticing traits, such as perpetual youth, general enhanced senses and abilities, and resistance to normal methods of death. But to be fascinating this is best balanced with a induced quality of life that would make most people not want to be bitten, and those that do later regret it, such as blood-lust and a sense of uncomfortable wrongness. Vampires are otherwise fully human in their emotions and feelings; you can imagine being a vampire and understand these resultantly evil creatures.
A similarly Lich was a person, who wanted immortality and magical power. A Lich is the pinnacle of obsession. But to become such, by balance, a magic wielder must sacrifice parts of his mind. What's left isn't what a normal human would want to be, but for someone obsessed: there wasn't much of those other parts anyway,
