Heironymus was an alchemist from the Middle Ages. He summoned the demon brothers Arzun and Tolay from the Netherworld to do his bidding, but instead, they brought him back with them. Centuries later, when Peter, Ray and Winston went to rescue Egon, Heironymus told them that Tolay captured him and took him to his castle. Heironymus asked to be taken back to the living world with them. As they escaped, Heironymus volunteered to stay behind because he feared that he wouldn't be ready for the major changes to Earth. So Winston gave him his proton pack so that he can fend for himself.