One of the single greatest achievements of the wizard Bonisagus was his invention of the Parma Magica [PAR-mah MAH-gi-kah]. It was one of the two great breakthroughs that made the Order of Hermes possible. Without something to protect them from other practitioners of Magic, wizards of the past were paranoid and mistrustful, often with good reason. The nature of The Gift itself and its affects on others lent to this feeling of paranoia. Encounters between wizards was more about self preservation than about the sharing of knowledge. Ancient Magical Societies and cults always caved in on themselves because no one could trust anyone else.
What Bonisagus discovered was a spell-like ritual that could be performed twice daily, at sunrise and at sunset. This ritual creates a magical resistence around a magus that acts as a shield against the powers of other magi. The nature of The Gift and its affects on other people is also suppressed by Parma Magica, so that magi could interact and share knowledge. The ritual takes only two minutes to perform and must be done at sunrise and sunset. Generally speaking, Hermetic Magi always have their Parma Magica up but they can suppress it momentarily by concentrating.
Although most associated with hermetic magic and the Order of Hermes, anyone with The Gift can learn how to perform the parma magica, even without any knowledge of the Hermetic arts. Because of its great power and versatility in magic defense, only those within the Order of Hermes are allowed to know it. "Join or die" would most especially apply to any Gifted practitioner who knew the ritual but was not yet a member.
- Ars Magica 5th Edition Core Rulebook - published by Atlas Games