Dr. Jacob Crowley was a scientist living in New York City who utterly despised Halloween. He was a very thin, frail-looking middle-aged man with a tall mishappen head which (apparently) housed an oversized brain. Due to his dislike of Halloween, Crowley invented a machine that could (through some unknown means) make everything related to Halloween disappear in an instant. He just needed one of Egon's PKE Meters to complete the device, which was stolen by his assistant Fairweather.
Little did Crowley suspect however that he was being used by a demonic entity called Boogaloo to open "the Halloween Door" and free thousands of ghosts, goblins, and other monsters, and that Fairweather was even one of them in disguise. With the Halloween Promise broken thanks to the disappearance of the holiday's trappings, Boogaloo and his minions were let loose into the city to wreak havoc, leaving it up to the Ghostbusters to destroy Crowley's machine, renew the Promise, and send Boogaloo back to where he came from.
Dr. Crowley is named after Aleister Crowley, a 20th century British occultist.