What are Epic Mysteries:
Epic Mysteries are mysteries developed from a core of various magical ingredients called seeds. Furthermore, they are made of pure shadowstuff directly from the plane of shadow, making them incredibly hard to eliminate. Despite their power, they still follow the rules of normal shadow magic unless otherwise noted. Much like with epic spells, however, it takes much time and personal investment to actually develop an epic mystery. Unlike with epic spellcasting however, rather than spending money and life force to develop their spells, they test their knowledge and power against the plane of shadow, putting their very life on the line.

Learning Epic Mysteries:
In order to learn Epic Mysteries, one only needs the epic shadowcasting feat. With that feat in tow, the two step procedure of development and casting can begin.

Epic Mystery Development:
Before it can be cast, an epic mystery must be developed. As shadows shift and morph less willingly than raw magic, it takes a mystery-user weeks to accomplish what a spellcaster could in days.
Every day that the mystery-user spends trying to learn the epic mystery, they must spend at least 8 hours studying and meditating, engaging in no more than light activity. If this is ignored for 3 consecutive days, they are set back by a week in their progress (or they must abandon the attempt and start again from scratch if they have made under a week of progress)
Every week, the mystery-user must make a knowledge (arcana) or knowledge (the planes) check against the mystery’s Spellcraft DC, in order to direct their studies properly. The mystery-user may not take 10 on these checks and they take a –2 penalty on the check for each day that week in which they did not prepare themselves. With each failure, no progress is made for the week. If the mystery-user fails their knowledge check on three consecutive weeks, they must abandon the attempt and start again from scratch.
In addition, once every 4 weeks, the mystery-user must bodily visit the plane of shadows and make a Spellcraft check against the mystery’s dc, to subtly shape the plane of shadows to their advantage. The mystery-user takes a –2 penalty on this check for each unsuccessful weekly knowledge check made within the last month and may not take 10 on such checks. If this Spellcraft check is failed, the mystery-user is slain, as they enter a realm of power beyond their control. If they are later revived from the dead, they must start again from scratch. An additional such Spellcraft check is made at the end of the development process in addition to that week’s knowledge check (if the development process ends at a 4-week interval, make only one spellcraft check, as normal).
Perhaps uniquely among shadow magic, the development of epic mysteries is so complex that it needs specialized tools, herbs, and other commodities, providing a tangible cost. This paltry fee, however, is as copper pieces compared to the fortunes needed to fund epic spell development. The specialized equipment can be ignored, if need be, but the mystery-user takes a –10 penalty on all knowledge and Spellcraft checks made to develop the epic mystery without the equipment.