Leshii

According to Sam's research in 5.05 Fallen Idol, the Leshii (also spelled Leshy, Leszy, pl. Leshie) is an Eastern European pagan god, the Master of the Forest. The Leshii is able to shapeshift. Farmers would make pacts with the Leshii to assure a good crop as well as protection.

In the episode, the god, whose forest in the Balkans had been cut down 30 years ago, takes on the form of celebrity idols and kills their worshippers, and plants seeds from that Balkan forest into the victims' stomachs. To kill the Leshii, Sam and Dean must cut off its head with an iron axe.

Particularly in Russian folklore, nature spirits such as the Leshii, the Vodianoi (water spirit), and the Polevoi (field spirit) were considered at best, capricious, and at worst, actively harmful to human beings; in post-Christian times, such spirits were often seen as manifestations of unclean or evil forces, and were often elided with folk concepts of the Devil. In the Olonets region of Russia, a Christian legend claims that when the Archangel Michael cast the rebellious angels from Heaven, the fallen angels became the spirits of the place where they fell: those that fell into forests became Leshie, those that fell into Hell became demons, those that fell into homes became Domovoi (house spirits), those that fell into water became Vodianoi, and so forth.

Leshie are great shapeshifters, and can take on the appearance of any animal or human; their usual representation is as a male human wearing the typical peasant dress of the region. Their remit is the forest and all who dwell or travel within it, but they are especially associated with wolves.

Contrary to the show, Leshie are not necessarily gods per se, but function in a manner analogous to certain fairies of the British Isles. Like fairies, Leshie lead travelers astray, drive interlopers out of their forests, and steal unattended children. Their usual method of killing is to tickle their victims to death (much like Rusalki). Those who had to travel in the forest protected themselves by turning their clothing inside out (a protection that also works against Western fairies), and by saying prayers before entering the forest. As the episode mentioned, some people entered into pacts or agreements with the Leshie, and were rewarded with protection and magical ability: hunters, and herdsmen whose cattle were pastured in the forest, were often reputed to have made such pacts. Other than such pacts, there does not appear to be any records of actual sacrifice to the Leshie, not even the small offerings of milk and bread that were sometimes left out for Western fairies -- unless one wishes to count the stolen children and murdered travelers as such. If this spirit is able to be killed, folklore has not recorded any information on how to do so.

For further information, see Russian Folk Belief by Linda Ivanits, and The Bathhouse at Midnight: Magic in Russia by W.F. Ryan.