Samhain is the Spirit of Halloween and a recurring enemy of the Ghostbusters in the cartoon series The Real Ghostbusters. His name is derived from the Feast of Samhain, an ancient Irish festival that would evolve over time to become the modern holiday of Halloween. Samhain is stated to be an unstoppable creature of the night, commanding an army of ghosts and other supernatural creatures. He believes that such creatures have been eternally mistreated by humanity and wishes to exact vengeance on humans by casting the world into everlasting night, allowing ghosts to rule.
Samhain dates back to seventh century Ireland, where Halloween started out as the Feast of Samhain. It was recorded that he was an unstoppable creature of the night. At some point he was trapped in stone. Samhain remained imprisoned in an ancient clock for centuries. In the 1980s, the clock was brought from Ireland to New York to be on display in a museum. As a result, P.K.E. activity spiked. Two weeks later, Samhain was released from his captivity by his two goblin minions on Halloween night. Once set free, he gathered all the ghosts he could and sent his minions all over New York. He also used his powers to halt the night as he planed to make Halloween an eternal night that will last forever all over the world. The Ghostbusters however ran all over town capturing his minions, to which Samhain confronted them out of rage. However, the Ghostbusters used his weakness against light to stun him. Egon came up with a plan to use all the lights in the city on Samhain but a blackout occurred. While Peter, Ray, and Winston distracted Samhain and his minions with a frontal assault, Egon set up spotlights around the building he perched atop. He was captured (along with his minions) just before the world fell under his evil reign forever. He was placed into the Containment Unit where it was assumed that he would stay.
For quite some time, Samhain remained in his prison. Yet despite getting along with his fellow inmates, he never enjoyed being imprisoned. The next year after Samhain was captured, Dib Devlin used an illusion of Samhain on Ray and Winston in the second part of his game show Race the Devil. It was a trick to fool the Ghostbusters into giving up. While revealing the fake Samhain, Dib said, "Why, it's an old friend of yours." "It's Samhain, the living embodiment of Halloween!" But during the battle, Ray realized they already captured the real Samhain. The real one was still in the Containment Unit. Once Ray realized this, he stood his ground and defeated the fake Samhain with his bare hands.
The next Halloween, Samhain was released from the Containment Unit thanks to his two goblins liberating him again. This time Samhain's primary goal was to take revenge on the Ghostbusters for his long imprisonment. He summoned more ghosts and quickly took over the Ghostbusters' headquarters, transforming it into his new fortress. The Ghostbusters managed to get everyone else and escaped just before the headquarters completely transformed. The Ghostbusters attempted to use his weakness to light again, but Samhain set up a protective barrier around the fortress that only other ghosts could pass through. Samhain then led his minions around the town to search for the Ghostbusters, allowing the Jr. Ghostbusters to investigate the fortress. The Junior Ghostbusters were chased by sentries and later caught by ghosts led by Samhain. The Ghostbusters used a camera hooked up to Slimer and saw that Samhain's fortress was held together by a keystone that was in exactly the same place that the Containment Unit was in. The Ghostbusters attacked Samhain in his fortress and blasted the stone from outside the fortress while Slimer attempted to save the kids. After the Jr. Ghostbusters got out, the keystone was destroyed, causing the entire fortress to crumble and unveil the Containment Unit (which was left open after Samhain escaped). With the Containment Unit still active, Samhain and his minions were sucked back into their prison.
As the Ghost of Halloween, Samhain is a very formidable foe for the Ghostbusters. He is legendary and a number of ghosts seem somewhat terrified of him. Samhain possesses the ability to halt time and cause eternal night. He has attempted to use this power at various times, but is always defeated by the Ghostbusters in the end and his spell is broken. Samhain also has the ability to control entire legions of ghosts, though it is unknown if he controls them himself or they obey him out of fear. The latter may be the most possible as Slimer seemed to drawn by force to him yet did not seem to want to obey him. Samhain also possess an ability to fire black lighting, as he fired one at Peter Venkman and the other Ghostbusters during their first encounter. His powers seem to be at their highest point on Halloween as the Ghostbusters were being overworked with a huge increase in ghost activity.
Samhain's only real weakness seems to be captivity, as it is the only way to prevent him from halting the night forever. While captive, Samhain is virtually powerless to escape, though he can be released should his prison come in contact with an infinity symbol. He also can be warded off by light as a number of ghosts can. This weakness was used against him during his first encounter with the Ghostbusters and they attempted to use it against him during his second coming.