In Norse mythology, the Vanir are originally a group of wild nature and fertility gods, the sworn enemies of the warrior gods of the Aesir. They were considered to be the bringers of health, youth, fertility, luck and wealth, and masters of magic, also known for protection and prosperity, and keeping the local settlements safe from harm. Some villages built effigies of the Vanir in their fields, while other villages practiced human sacrifices consisting of one male and one female.
The Vanir is a pagan god that resides in an ancient tree brought to the U.S. by Scandinavian immigrants. Each year the people of Burkitsville, Indiana present it with a young man and woman as a sacrifice to ensure the prosperity of their town and its crops. The god inhabits the form of a scarecrow made from the body of its previous victims and kills the couple with a hooked blade, or scythe.
After Dean saves a couple from being attacked by a scarecrow, Dean calls Sam to tell him of the attack, and quickly surmise that he must be dealing with a pagan god rather than a spirit. He believes this due to the annual cycle of its killings, compounded with the fact that the victims are couples -- which may mean it is for a fertility rite.
Dean then heads to a community college where he speaks with a professor, and learns he is dealing with a Vanir and that it's power may come from a tree. However before Dean can put a stop to the Vanir, he is captured by the Jorgeson's and the town sheriff and placed in a cellar, where he is soon joined by Emily, who tells him about the town's ancestors bringing over a sacred tree known as the First Tree.
Before the Vanir can take its sacrifice Sam arrives and unties Dean and Emily, while Sam wants to find the sacred tree Dean decides they should wait till morning, lest they want to cross paths with the Scarecrow. The next morning Sam, Dean and Emily find the First Tree, which she gladly burns -- vanquishing the god.
Vanir in Lore
Freyja, Freyr, and Njord were the usually-named Vanir deities. In actuality, while there was conflict between the Vanir and the Aesir, the two groups intermarried. While the ancient Germanic and Scandinavian tribes did practice human sacrifice, surviving evidence indicates that the god Odin/Woden/Wotan, of the Aesir, was the most common recipient of human offerings.