Samhain

Samhain first appeared in seasons 1 of The Real Ghostbusters in episode #8 "When Halloween Was Forever"
Samhain is a pure embodiment of Halloween, and the self-proclaimed ruler of the night. His origins can be traced back to 7th century, Ireland. According to legend, this was where Halloween first began with the Feast of Samhain. Samhain's presence envelopes the world in a never-ending darkness, wherein time stops completely. It was said that no one could stop him, but he was subdued. His spirit was trapped in a massive tomb, where he remained dormant until 1986.
Samhain's ruins were discovered and brought to New York to be placed on display at a museum. A pair of gremlins discovered Samhain's container there, and together they released him from his prison to wreak havoc on the world once again.
The Ghostbusters quickly mobilized to stop him. They discovered that his weakness is light, and managed to trap him with the use of massive search lights. He also appeared in season 2, episode #85 "Halloween II 1/2" One year later, the same gremlins that first released Samhain intruded into the firehouse. Their presence went unnoticed, as the Ghostbusters were amidst a large costume party to celebrate Halloween. They freed Samhain from the containment unit. Samhain announced that his first priority was to carry out a vendetta on the Ghostbusters. He proceeded to transform their headquarters into a massive, impenetrable fortress of his own design.
With the help of the Junior Ghostbusters, Samhain was defeated once again.