The Real Ghostbusters: Ragnarok and Roll

Season 2, episode 16, in which a "depressed young man uses a magical flute to bring about the end of the world".

"Ragnarok and Roll" has to be like, my favorite play on words EVER.

The Real Ghostbusters: Sandman

"Mr. Sandman, Dream Me a Dream" (Episode #107, aired 10/25/1986):

The Sandman arrives in New York and attempts to put everyone to sleep for five hundred years. And with every person put to sleep, their dreams become real. Can the Ghostbusters stop the Sandman or will they fall asleep with the rest of the city?

The Real Ghostbusters: Samhain or Sam Hain?

"One of the only recurring villains is Samhain. He first appears in the episode "When Halloween was Forever". In the episode, he is released from his captivity by two goblins on Halloween. Once set free, he sends his minions all over New York. His plan is to make Halloween an eternal night that will last forever all over the world. Just before the world falls under his evil reign, the Ghostbusters put a stop to him and his terror. He is placed within the ecto-containment unit where it is assumed that he would stay."

-Wikipedia