Petrus was a member of the Order of Hermes (house not stated) and a former colleague of Reginald Wojcik. The two of them trained together, and so when Wojcik became a Nephandus, the Order asked Petrus to oversee the operation to bring him in.
Petrus delegated most of the legwork to Zydeco Jones and Card Farrel (and later, Ozymandias Cody) while he operated in the background, feeding them information through a series of dead drops located in isolated desert mailboxes. When Cody complained of the strain the work was causing to his marriage, Petrus blithely advised him to leave his wife; Cody noted at the time that Petrus spend more time in various Horizon Realms than on Earth.
Petrus disappeared into the Umbra while attempting to travel through the Gauntlet to his home in Crete after the Avatar Storm.