Samhain is the main antagonist of the season four Supernatural Halloween-themed episode "It's the Great Pumpkin, Sam Winchester".
In a small town in America, two witches were preparing a spell to summon the ancient demon Samhain, whose name is the original Halloween festival on 31 October. Samhain was in fact a demon, originally a human who had done some evil deed and been banished to Hell. However, he had control over the Undead, and could bring forth all manner of monsters.
Samhain was being prepared for freedom by the two witches, and Michael sent down two angels, Castiel and Uriel to deal with the problem, under the pretense that the angels did not want Lucifer free. In fact, summoning Samhain was one of the 66 Seals needed to free Lucifer. So, the witches were preparing to free Lucifer. The Winchesters persuaded a warmongering Uriel not to destroy the town but to spare the people in it when they fought the witch. However, the witch turned out to be a teacher named Don Harding, who was apparently sacrificing his student, Tracy Davis, and the brothers intervened to save Tracy and killed Don. Before Tracy turned on them revealing herself as an ageless witch. With Don's death they freed Samhain, Samhain possessed Don. Don's corpse rose and greeted her, but then killed her by snapping her neck and dismissed his loyal follower as a "whore." Sam and Dean managed to survive by covering their faces in blood (Dean remembered how Samhain's weakness was that he couldn't see a human if their faces were masked).
Proceeding to the cemetery where a Halloween party was, Samhain used Don's body to fool all the students and locked the gates on them, then summoned zombies to kill them. They managed to kill a few of the children but the brothers opened the gates and saved the others. Dean then fought all the Undead creatures that came forth, while Sam perused Samhain. Trapping him in a coridor Samhain tried to destroy Sam with a blast of his demonic power, thankfully Sam was immune. Enraged Samhain attacked, Sam battled Samhain but the demon proved too strong, as Sam had lost the demon killing knife and out of options, Sam was forced to fight using his demonic powers, despite Castiel having said he should stop using his powers. Through great strain and pain he managed to exorcise the demon and send Samhain back to Hell.