The concept of psychical vampire.
A person or entity who drains life energy from a human victim, most typically without their knowledge and nearly always without their consent.
Two categories: unintentional and intentional.
Unintentional the most common. Typically a person who is physically unwell, for instance a sufferer from a terminal or other serious disease. Unknowingly they drain the psychic energy of those around them, most probably driven by carnal panic in the face of death, in any case because they physically sense the need for extra energy necessary to survive. By implication, those who are emotionally or mentally unwell, driven by unconscious fears to similar results.
Intentional the most dangerous. Typically individuals who practice psychical or magical techniques for feeding from the energy of others, sometimes under group guidance, sometimes from simple power lust, sometimes with the mystical intention of achieving a bent form of spiritual immortality.
There are no physical defenses. Crosses, garlic stakes through the heart, etc.
It's said that powerful ritual defense is straightforward, requiring only four simple steps which sound easy to master. 1) Ritual purification of self, particularly the astral self, removing negativities; 2) Ritual banishment from an area, creating a circle of safety; 3) Ritual removal of / separation from psychical links left behind by any possible previous attacks; 4) Ritual programming of the astral body to act as a spiritual shield against malevolence from outside. Physical ritual steps are simply aids to concentration; the work takes place entirely in the imagination.
See Konstantinos' interesting book, Vampires, The Occult Truth.