Hieronymous (The Masque of Mandragora)

Hieronymous was the official astrologer of the Court of San Martino in the 15th century. Secretly, he was also the leader of the Brotherhood of Demnos.

Biography
He appeared to give his fealty to Count Federico, and sometimes cast his horoscopes so as to "predict" murders that Federico's men then carried out. Indeed, he was involved with 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.

However, Hieronymous was not actually working for Federico so much as himself. He believed than the Botherhood of Demnos would eventually reveal itself the true power in San Martino. His actual horoscopes predicted an entirely different power struggle than the mere 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 also 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, thus preventing at least European emergence from the Dark Ages. The plan would have succeeded had not the the Fourth Doctor not intervened. In the end, Hieronymous himself was killed by the very Helix energy he sought to tame. (DW: The Masque of Mandragora)