He was a demon that was trapped by the Ghostbusters after Egon used his latest invention to make him incorporeal.
In 1690, Heironymus attempted to summon both the demon brothers Arzun and Tolay from the Netherworld to do his bidding. He failed and was pulled into their realm.
Centuries later, Arzun crossed over to the physical realm and arrived in New York City. The Ghostbusters engaged Arzun and Egon took the opportunity to use his latest invention, the Destabilizer and discorporate the demon.
However, the Destabilizer's beam was redirected and hit Egon. As part of a proto-ectoplasmic conversion, Egon vanishes. The rest of the team regrouped and trapped Arzun. His brother Tolay faced the Ghostbusters when they entered the Netherworld to find Egon.