A Vampire is a nocturnal creature which subsists on blood. Vampires are regarded as emissaries of Satan and are thus afflicted by consecrated ground or water, as well as by symbols representing Jehovah or Christ such as the crucifix. Vampires have inherited both immortality as well as an aversion to fire and sunlight.
As preternatural predators of men, vampires are resilient, quick, and inhumanly strong; able to easily outrun and kill any man in seconds as well as quickly recover from grievous wounds and even regrow severed limbs short of decapitation, a stake through the heart, or immolation.
As they grow older, they are stated to become both stronger and more disciplined. Elder vampires, those with centuries behind them, are chronicled as having walked upon church grounds unhindered as well as endured both blessed water and crosses without showing visible discomfort.
Originally human, vampires are made - or turned - presumably through siring; a vampire feeds upon a vessel nearly to the moment of total exsanguination, the sire then must feed the victim its own blood both to seal the bloodline and to keep their heart beating for the transition.
Vampires are outwardly indistinct from humans, save for irises which shine sanguine in the dark and a pallid complexion due to lack of exposure to daylight. When roused or hungry, their eyes become bloodshot and their fangs emerge; the maxillary canines sharpening to fine points.
It's possible that their saliva may possess anticoagulant and preservative properties. This would aid in their feedings as they would have easy access to the blood-flow without worrying about clotting or unwanted blood-loss, both of which could kill the vessel if left unattended.
The crux of a vampire's existence is the blood flowing through their veins. Without a steady stream in their system, vampires will both biologically weaken as well as psychologically degrade; their strength and senses may increase, but their healing and mental faculties diminish.
If starved of blood for prolonged periods, a vampire's appearance will degrade; unable to maintain a human appearance and becoming more predatory as their hunger grows and their metabolism tries to compensate for it deficiency of nourishment by sharpening their bestial instincts.
Although they possess the same intellect and overall persona as they did in their previous lives, vampires are far more instinctual and aggressive than men; having been reborn as solitary hunters, usually attacking animals or even other vampires who encroach upon their territory.
Despite their territorial natures, some vampires may come together to form a coven; a group of several, younger vampires led by a single, older vampire. Covens are effective both for protection and hunting, as no recorded victim has ever survived an encounter with a hungry coven.
As they are barred from entering the afterlife, the souls of slain vampires presumably remain on Earth in a state of limbo behind the veil between life and death. These spirits may occupy the ranks of ghosts or possess corpses to rise as revenants until the body decays into dust.