Source: M.A.Lloyd, based on an idea from Jeff Wilson
Mysteries are extremely powerful effects, often unsuitable for subtle magic campaigns, but appearing in some stories. Learning a Mystery should never be simply a matter of spending points. If the GM decides the Mystery exists at all, and that the PCs can learn it, he should make sure the process is a memorable event in the campaign. Perhaps it requires a long search for a teacher or a difficult vision quest to discern the principles which must be played out.
Unlike rituals, Mysteries are not based directly on Path skill; each one must be learned as a separate (Will/Very Hard) spell. They are associated with one or more Paths though, which must be known at 20 or better to comprehend the Mystery. Mysteries are the equivalents of cinematic skills, and the mage may need a 40 point Enlightened advantage (equivalent to Trained by a Master) to learn them.
Mysteries also tend to have more restrictive casting conditions than rituals. A ritual can be cast at a penalty under unfavorable conditions, but a Mystery usually will not work at all. Most Mysteries can only be cast at certain times (at dawn, under a full moon, 'when the stars are right') require specific components (now for this we'll need a black bull sacrifice, the hand of a virgin murdered by her younger brother, and 6 fresh albino pineapples...) or only work at particular places, often to something more unusual than just 'consecrated ground' (within stone circles, on former battlefields, beneath a gallows at midnight on all Hallows Eve). The GM may set any conditions that seem appropriate, though if certain elements are required the casting bonuses for them should still apply.