William Parvus, better known as William of Newburgh, was a 12th century Augustinian cannon regular and an English historian who lived from 1136-1198 according to most records. William of Newburgh wrote many statements on the undead, and his works are regarded as some of the most informative and extensive records of the vampire phenomena to date. Some of the earliest known stories about vampires and vampire hunters can be found in his most famous work of non-fiction, Historia Rerum Anglicarum or the History of English Affairs during the Middle Ages.
It may be important to note here that the word "Vampire" didn't enter the English language until 1734 (for more on that subject, read my article on the Writing forum titled Derivation of the English Word: "Vampire" as I go into more detail about the origins of the word). William of Newburgh and his contemporaries didn't have a word for vampires during the 12th century. So they refered to them as "revenants," from the Latin reveniens meaning "returning" or the French revenir meaning "to come back." That is because it was believed with utmost certainty that the corpses of those who had died were in some cases being animated back from the dead. Thus the vampire epic was born.
William writes on one of his journals in 1191 A.D.:
"It would not be easy to believe that the corpses of the dead should sally (I know not by what agency) from their graves, and should wander about to the terror or destruction of the living, and again return to the tomb, which of its own accord spontaneously opened to receive them, did not frequent examples, occurring in our own times, suffice to establish this fact, to the truth of which there is abundant testimony."William of Newburgh didn't stop there. He is known to have written several different accounts of these revenants as they appeared in different English villages. William wrote these cases according to his own words, "... as a warning to posterity." These were not fairytales or children's stories, but writings based on professional testimony and eyewitness accounts by doctors, lawyers, professors and officials of good standing within the court and church. William wrote that the occurrences of the undead rising from their graves were so many that, "... were I to write down all the instances of this kind which I have ascertained to have befallen in our times, the undertaking would be beyond measure laborious and troublesome." It's no doubt that people during the 12th century held a very strong and frightening belief in vampires.