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Ghoul (folklore)
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This term has other meanings, see Ghoul.
Ghoul is a mythical creature, a corpse-eater. According to the tales and legends of Central Asia, it is absolutely harmless, lives in caves and wells, on battlefields. In the latter, it appears along with ravens. It is afraid of and avoids living people. It has a terrible appearance, bluish, dead, slimy skin.

In the 20th century, ghouls became characters in literary works in the fantasy genre, thanks to Robert Howard's widely known "Hour of the Dragon." There, ghouls not only devour carrion but also attack even armed people. Sapkowski creatively developed this theme, introducing a new type of scavenger in his works — graveyards, which are more dangerous than ghouls. Here is how he describes both:

“A ghoul, Uncle Vesemir, is a monster that devours corpses. You can find it in cemeteries, near burial mounds, everywhere where the dead are buried.”
“So, is it dangerous only for the dead, or what?”
“Not only. A ghoul also attacks the living. When it is hungry or enraged.”
“You remembered correctly, clever girl. Now, look at this engraving.”
“Uuuu…”
“I agree. This creature is not beautiful. This is a graveyard. A graveyard is a type of ghoul. It is very similar to a ghoul, only much larger. As you can see, it is also distinguished by three bony ridges on its skull. The rest is like any corpse-eater. Note: short, blunt claws, suitable for tearing graves… Powerful teeth with which it crushes bones, and a long, thin tongue to lick out the decomposed brain and fat. Stale fat is a delicacy for a graveyard…”
“What’s wrong with you?”
“Nnnnothing…”

The creators of the game "The Witcher" went even further, inventing a more powerful and repulsive monster — a cemataur.

[edit] Mentions in literature
“The Tale of Saif al-Muluk” from the cycle “One Thousand and One Nights.”
Robert Howard. “Hour of the Dragon.”
Andrzej Sapkowski. “Sword of Destiny.”
Andrzej Sapkowski. “Blood of Elves.”
Andrey Belyanin. “Baghdad Thief.”
Howard Lovecraft.

Added 8 minutes ago
Vurdalak is a mythical creature, a living dead, usually resurrected from dead people or from those who were bitten by another vurdalak. They drink the blood of their relatives; usually, entire villages die out due to the appearance of vurdalaks, as they kill primarily, turning them also into vurdalaks, the closest people (daughter, mother, father, etc.).

There is also an opinion that a vampire, biting a person, thereby transmits part of its curse to the victim and turns the person into a vurdalak, who performs various orders for its master.

Sometimes vurdalaks are identified with werewolves. It is believed (see, for example, “Etymological Dictionary” by M. Fasmer) that the word “vurdalak” itself arose by chance: A.S. Pushkin created the poem “Vurdalak,” incorrectly conveying the word “werewolf” (possibly influenced by forms such as Bulgarian върколак).

Added 2 minutes ago
Upyr in Slavic mythology is a living dead, killing people and sucking their blood or eating them. Also, this word can be used to refer to an evil and hostile person.

According to popular beliefs, upyrs are wandering dead who were werewolves, sorcerers, or were excommunicated from the church and anathematized (heretics, apostates, some criminals, such as serial killers, etc.) during their lifetime. At night, upyrs rise from their graves and walk the earth, thanks to their human-like appearance, they easily enter houses and suck the blood of sleeping people (and feed on it), then return to their graves — обязательно before the third rooster crows. According to popular belief, an upyr could be killed by piercing its corpse with an aspen stake. If this did not help, then the corpse was usually burned. It was believed that upyrs cause plagues, crop failures, and droughts.

Upyr approximately corresponds to a vampire in European mythology and has much in common with a vurdalak in the East Slavic tradition, however, in the 19th century, in the popular consciousness, these characters were clearly differentiated.

The opposite of upyrs in archaic tradition is the beregini, who protect a person, the personification of all that is good.

Now, when everyone has access to the myths of many peoples, and when many fantasy worlds have emerged, an upyr is described somewhat differently. Any person (except for the most righteous) who died an unnatural death and was not buried properly can become an upyr. An upyr is a lower undead, but, unlike skeletons and zombies, it can arise on its own (while those must be created), it has some degree of reason and free will, an instinct for self-preservation, etc. An upyr reproduces by leaving not completely gnawed corpses of its victims, which become (rise) new upyrs. A new upyr does not depend on the old one; several upyrs that have risen together form a pack. Most often, upyrs appear on the site of battles, and if they are not exterminated, they can soon form an army of several hundred individuals. The strongest upyrs eventually turn into vurdalaks.

Added 1 hour 31 minutes ago
Vampires are a mythological or folkloric monster. From all angles, vampires are undead, corpses that feed on human and/or animal blood. They are also a frequent object of film or fiction, although vampires from fictional works have acquired some differences from mythological vampires (see Characteristic features of vampires from fictional works). In folklore, the term is usually used in relation to a blood-sucking creature from Eastern European legends, but vampires are often called similar creatures from other countries and cultures. The characteristic features of a vampire in various legends vary greatly. Some cultures have stories about non-human vampires, such as bats, dogs, and spiders.

In zoology and botany, the term “vampirism” is used in relation to leeches, mistletoe, vampire bats, and other organisms that feed on the bodily fluids of other creatures. The term is also used in relation to cryptozoology and blood-drinking creatures, including the chupacabra.

The term “vampire” can be used to refer to any mythological magical creature that is a predatory parasite, sucking the strength, energy, or life from victims. Mythological creatures that act in a similar way are often considered a model of a vampire, even if they do not enjoy blood.