Vampires

"The vampire spawn ran through the streets of Vallaki, spreading terror in their wake. A passing woman screamed as one fell on her, mauling her throat and laughing as blood spattered across the cobblestones. The town bell rang mournfully as panic set in among the crowds, villagers running every which way under a sky that seemed to turn to night all too quickly. A figure upon a black steed flew across the sky, cloak billowing, and in the light of a baleful moon glowed a handsome, chiseled face. A slight smile came to his face as he oversaw his thralls' handiwork."

- The Curse of Strahd

Tales of blood-drinking creatures have been used by weary mothers to scare children since time immemorial. But only an unhappy few know the truth: that vampires are real, and terrorize parts of the Known World. These knowledgeable folk pray that a rogue vampire never takes residence in an abandoned manor or a nearby ruin. For places under the thrall of vampires invariably become corrupted, the night itself seeming to swallow up villagers one by one, until only a terrified few remain...
Vampires are some of the most dangerous creatures ever to walk the world of Erden. Neither truly undead nor truly alive, they possess superhuman strength, speed, and agility. When sated with the blood of sentient creatures, they are resistant to damage and have the ability to regenerate wounds in seconds. But worst of all, they are highly intelligent, having centuries of experience fighting and learning the ancient secrets of the world.
Vampires are not one entity, but rather a collection of related creatures that become more powerful from consuming blood. The weakest, a vampire spawn, may be a threat to a small village or hamlet only. Conversely, a single higher vampire may be a threat to empires, being able to command armies, assassinate enemy commanders, and use magics of nearly incomprehensible power. Vampiroids are not true vampires, but refer to a group of non-humanoid monsters that benefit from drinking blood. Vampiroids include the gheists, bruxae, vargheists, varghulfs, and other blood-drinking abominations.
The Path to Vampirism:
There are two known paths to vampirism. The first is to be directly converted to a vampire by the dread god Sanguine. People who attain this gift are considered first generation higher vampires, and are immediately granted great powers and vulnerabilities. These higher vampires gain superhuman attributes, true immortality, shapeshifting abilities, and massive regeneration. Conversely, they are vulnerable to radiant damage and are weakened in sunlight. Strahd von Zarovich is a notable example of an extremely powerful higher vampire.
The second path is to be converted to a vampire spawn by a higher vampire after being brought to near-death by a bite. A vampire spawn is much weaker than a higher vampire, being barely more powerful than a regular human when not blood drunk. Normally, a vampire spawn can subsist on the blood of animals. When fed humanoid blood, however, the vampire spawn becomes blood drunk. In this state, the spawn regenerates and is resistant to most damage.
History:
Vampires haunt the chronicles of the world, inflicting sudden catastrophes before fading into the shadows. Their legends are found in all known cultures and religions, yet accurate knowledge of vampires is limited among the smallfolk, perhaps because the truth is too horrible to contemplate for the average world-weary peasant. Many civilizations have crumbled due to vampire influence, their people dying out, becoming sentient cattle - or worse. The most notable example of this is Barovia, a realm so foul that none who enter it leave. Barovia was conquered by Strahd von Zarovich in the 300s, and has been a bastion of evil ever since.
After Strahd's conquest of Barovia, vampires faded into the background of history, for a time. But in the waning years of the 6th century, a vampire lord, Baron von Metus, began to terrorize the country of Lornham. He kidnapped numerous slumdwellers from the squalid alleys and asylums of Darkon, and it was only after many decades that his reign of terror was ended by the van Richtens. Castle von Metus remains a haunted ruin to this day.
Most recently, vampires fought alongside the forces of Sanguine during the War of the Broken Crypt. One of their highest ranking members, Lord Wachter - a Barovian - seized control of Carovin for many months.
Not all vampiric activity is so destructive as this. Vampires are very capable of producing things of beauty - orchestral music, imposing monuments, and works of art. Even fell Ravenloft is decorated with beautiful artwork, and Strahd himself is said to be a connoisseur of fine wines and a skilled musician. Nonetheless, these facts mean little to kidnapped prisoners exsanguinating on the cold floor of a vampire's torture chamber.

Vampiroids:

"A churning struggle played out in the muddy field before the battlements and the gate of old Barrowgarde. The shield wall was beset by stooping, bat-like monstrosities with claws like daggers, their maws dripping blood. More of the beasts climbed the crumbling wall with surprising speed: in a few more moments, the archers would be overtaken and devoured."

Vampiroids, sometimes called "creatures of the night", refers to any member of the coterie of horrible abominations that share some relation to the vampires. More animal than sentient being, these bloodthirsty creatures would rather charge into a group of terrified foes and tear them limb from limb than bite the neck of a princess. Many of them bear a resemblance to ogre-sized flightless bats, adding to their dreadful aspect.
Vampires and the Undead
It is a common belief among scholars that vampires are undead, but this is not strictly true. In truth, there is a line between vampirism and undeath, albeit a blurred one. True, vampires share many traits with the undead: they are immortal, have many of the same weaknesses, and are even sustained by necromantic energy. Also, vampires often use the undead as allies. But vampires also have many distinctions: they bleed, they require sustenance (through blood), and benefit from healing magics. Vampires likely occupy a middle-ground, being living creatures sustained indefinitely by necromantic magic generated from living blood.