A vampire is a being from folklore that subsists by feeding on the vital force of the living, mostly their blood. In European folklore, vampires are undead beings that often visited loved ones and caused mischief or deaths in the neighbourhoods they inhabited when they were alive. They wore shrouds and were often described as bloated and of ruddy or dark countenance, markedly different from today’s gaunt, pale vampire which dates from the early 19th century. In Dungeons & Dragons, the vampire is an undead humanoid (or monstrous humanoid) creature that looks as it did in life, with pale skin, haunting red eyes, and a feral cast to its features. The vampire first apeared in the «white box» set (1974), for the very first edition of the game.
A vampire retains all the abilities it had in life, plus it gains the ability to drain blood and life energy, and to dominate other creatures with its gaze. A vampire can also command rats, bats, and wolves, or take the form of those creatures. They also become superhumanly strong, can heal quickly or even regenerate, and can turn into a gaseous form.
Vampire spawn are undead creatures created when a vampire slays a mortal in a way other than with its bite. Vampire spawn look much the way they did in life (they are usually humanoids), except with hardened features and a predatory look. They also have greyer skin. They are very similar to vampires, being evil creatures drawn to their graves and coffins. They are much more brutish, stupid and while still strong, are physically weaker than vampires. They cannot turn into a bat, dire bat, wolf, or dire wolf and they cannot summon rats, bats or wolves, and they are unable to create spawn of their own.
The nation of Karrnath bears a lot of similarity to the land featured in «Dracula» by Bram Stoker. Karrnath bears a distinct Transylvanian feeling with a lingering aura of darkness, thick pine forests and a burgeoning middle class. Indeed the correlations of a vampire king lording over a military dictatorship holds sway with the imagery of Stoker’s work.
Besides Karrnath, the faith of the Blood of Vol is based on the idea that the undead are champions of the church. Keith Baker deepens a little about these particular vampires in its Eberron Expanded article about Libris Mortis, a 3.5e sourcebook, saying that worshipers willingly give their blood to vampires to strengthen them in battle. According to Libris Mortis, the greatest force driving a vampire is its inescapable craving for life energy, a hunger far more lethal to the victim than a little gift of blood. Such a system of sustenance represents a considerable sacrifice on the part of the faithful, and one that would quickly decimate Blood of Vol sects. Thus, vampires associated with the Blood of Vol should have a lesser craving for life energy. While they still require it to survive, the satiation period is less frequent and its damage is also reduced.