Wikipedia
A Krsnik or Kresnik is a type of vampire hunter, a shaman whose spirit wanders from the body in the form of an animal. The krsnik turns into an animal at night to fight off the kudlak, his evil vampire antithesis, with the krsnik appearing as a white animal and the kudlak as a black one. The krsnik 's soul leaves the body, either voluntarily or due to a higher power, to fight evil agents and ensure good harvest, health, and happiness.
The legend evolved from a pre-Christian myth present in Slovenia, Croatia (mainly Istria and the islands), and other countries, in which the celestial pagan god Perun is locked in eternal combat with the evil snake of the underworld, Veles. The krsnik is taught magic by Vile (fairies), and in traditional medicine has the ability to heal people and cattle. However, due to the undocumented nature of oral tradition, it's difficult to determine with certainty how much of kresnik folklore originated from Slavic mythology ( Kresnik (deity)), and how much arose from a separate shamanistic tradition. After Christianization, the kresnik instead was claimed to have learned magic at the School of Black Magic in Babylon, but retained benevolent traits as a generous and powerful friend of the poor.
The origin of the name may be from the word krst, which means "cross", and which in Serbia is the word for a stone sign denoting village boundaries. It may also be derived from the same root as the Slav word for "resurrection," so that the word itself means something approximating "resurrector."
Similar beliefs circulated among the Italian Benandanti cult. Also see zduhać.