Countess Mircalla Karnstien is the name of a vampire evidently native to the Austrian Empire who died sometime after 1698. J. Sheridan Le Fanu wrote an account of her titled Carmilla published in 1872. Internal evidence suggests the story took place between 1835 and 1846.
Other accounts of her life abound, including references in several motion pictures, more than one novel as well as animes and comic books. Making the matter thornier is that the Karnsteins may have re-used the name Mircalla as well as anagrams of same (such as Carmilla or Marcilla) multiple times and more than one of these seem to have been vampires.
John A. Small makes the argument in "Kiss of the Vampire" (below) that the figure in Le Fanu's story was in fact Carmelita Rodriguez, a victim of Dracula (and sire of Vampirella), that the events took place in Spain but were transplanted to Austria for ficitonalization purposes. It should be pointed out that some evidence exists from cinematic chronicles (The Blood Spattered Bride as well as the poorly-named Daughter of Dracula) that a Spanish branch of the Karnsteins did exist.
Charles Loridans in "Children of the Night" (below) places the story of Carmilla as taking place in 1871, despite the narrator's claim the events took place at least a decade previously. In the novel, mention is made of a summer party attended by "Archduke Charles" which would seem to be "Charles of Austria" who died in the spring of 1847, making 1846 the lastest date it could have taken place.