- You may be looking for the Pharisee of the same name.
He appeared to give his fealty to Count Federico, and would sometimes cast horoscopes to "predict" murders which Federico's men then carried out. Indeed, he was involved in a partially successful plot to foretell the deaths of Federico's brother and nephew. Had the prediction come true, and Federico's nephew died, Federico would have inherited the Duchy of San Martino. In the end, however, he only correctly predicted the death of Federico's brother.
Hieronymous was not working for Federico so much as himself. He believed the Brotherhood of Demnos would eventually reveal itself as the true power in San Martino. His actual horoscopes predicted a power struggle entirely different from the political one being waged between Federico and Giuliano. He saw a day when Demnos would awaken and bring much more than just San Martino into its hegemony. This prediction was partially correct. The Mandragora Helix visited its power upon Hieronymous, and would have used Hieronymous as a tool to kill key figures of the Renaissance, preventing at least European emergence from the Dark Ages. The plan would have succeeded had the Fourth Doctor not intervened. In the end, Hieronymous himself was killed by the very Helix energy he sought to tame. (TV: The Masque of Mandragora)