Here's a sketch of Mirror Magic before I include it with the Mirage supplement--just in case someone notices that the idea *isn't* so brilliant, or carries with it a terrible flaw. Critique away, you bastards!
A few notes, since this is the rough draft--BaB progression is the same as wizard, as are saves. Mirror magic can be scribed on scrolls and behaves *identical* to normal magic--the situation in which it works is just different. And yes--Mirror Magic works FINE in antimagic shells. Any situation where normal spells fizzle, Mirror Magic works. Any situation where normal spells work, Mirror Magic fizzles.
Mirror, Mirror, on the wall
Who's the sharpest of them all?
"To understand mirror-magic, you must first understand the Law of Opposition."
"Everything in the Multiverse is balanced; everything possesses an opposite. Law to Chaos, Good to Evil, Nothingness to Infinite. Between these forces lurks the murky ground of contradiction--Neutrality, where the two forces wage eternal war. This is the Rule of Three--pay heed, for it governs the Multiverse."
"And yet with magic, we only have two sides of the coin--Magic and No-Magic. To the untrained eye, this fulfills the Law of Opposition but breaks the Rule of Three; but Mirror-Mages know better. It defies both rules."
"The opposite of Magic is not No-Magic. It is Mirror-Magic. Where Magic rules, Mirror-Magic fizzles; where Magic dies, Mirror-Magic flourishes. On the fringes of the Outlands, Mirror-Magic cannot so much as summon a cricket--but at the base of the Spire itself, it can unleash the maelstrom."
Of all the considerable magic that the Multiverse has to offer, Mirror-Magic is the most contradictory. In all places where normal magic ceases to function, Mirror-Magic thrives--and in all places where normal magic does work, Mirror-Magic fizzles.
Practiced almost exclusively by a small group of radical Kamerel, Mirror-Magic requires a specialized Prestige Class as well as 'Mirrored Spells'--spells that considered 'reversed' versions of normal spells. Otherwise identical to their normal counterparts, mirrored spells only function in places of limited or suppressed magic--and can only be memorized by a Mirror-Mage. The radical sect of kamerel, who have recently rebuilt a limited version of the 'Mirrored Library', have unlimited access to mirrored spells (all spells recorded in the library are automatically mirrored); other characters will likely need to go to extensive lengths to either retrieve these spells or research them on their own. Sorcerors cannot learn Mirrored Spells (except for Nerra Sorcerors, who can only learn Mirrored Spells).
There are whispered rumors of a mirror version of divine magic that allows a cleric to dedicate themselves to the denounciation of a specific God; however, these rumors remain unsubstantiated despite the Athar's recent attempts to learn more.
To qualify to become a Mirror-Mage, a character must fulfill all the following criteria.
Ability to cast 4th level arcane spells.
Spellcraft rank of 10 or higher.
The Mirror-Mage’s class skills (and the key ability for each skill) are Concentration (Con), Craft (Int), Decipher Script (Int), Knowledge (all skills, taken individually) (Int), Profession (Wis), and Spellcraft (Int).
Skill Points at Each Additional Level
2 + Int modifier.
All of the following are Class Features of the mirror-mage prestige class.
Weapons and Armor Proficiency
A mirror-mage gains no proficiencies in armor, shields, or weapons.
At 1st level, a mirror mage may research mirror-versions of any spell they can cast. The mirror mage must have a copy of the spell on hand (either in their spellbook or a scroll) and must spend one day per spell-level in uninterrupted study, making a Spellcraft check at the end of this period to determine their success (DC: 20 + 1 per spell level). If the mirror mage succeeds, they may scribe the reversed mirror-version of their spell into their spellbook as normal.
At 2nd level, a mirror mage gains the ability to memorize a mirror-version of a spell of 1st level. At each additional class level after 2nd, a mirror mage may memorize a mirror-version of a spell one level higher (to a maximum of level 9 spells at level 10). Mirror-spells occupy a spell-slot just like an ordinary spell, and are for all intensive purposes treated identically to normal spells--the only key difference is when they function.
Finally, how does Mirror Magic work in the Outlands? Simple--if you can't cast a normal spell, you can cast its mirror-magic spell. In the 9th ring, you can cast level 9 Mirror Magic spells; in the first ring, you can cast level 1 Mirror Magic spells.
Depending on how this screws things up, I might want to reverse it (though that kind of breaks the theme of Mirror Magic only working when NORMAL magic doesn't), although I don't think there's much wrong with this setup. It's a little odd and maybe a bit anti-intuitive, but thematically I think it's perfect.
A few other important questions: Why didn't the Kamerel KICK THE RILMANI'S FRIGGING ASS if they had this? Simple--the Kamerel needed the Mirrored Library to do this really well. Most Kamerel never bothered going any further than 1 level into this PrC; maybe two. Gaining the research notes for higher end Mirror Magic spells was just too difficult for the xenophobic race.
A radical group of Kamerel have built their own Mirror'd Library, though--one that only creates mirror'd spells. THESE guys are dangerous as Hell, and probably major movers in the game. In addition, some of the doppelgangers have gained one or two levels in this PrC--its how they're operating a lot of the Kamerel city (which runs on low-level Mirror Magic).
Why can't people get really high Mirror-Magic levels? I don't... Know. Maybe there should be some sort of pre-requisite for REALLY high levels in Mirror-Magic--or maybe you can't research Mirror-spells (You have to gain them from the Kamerel Mirror Library, which REALLY makes this PrC a bitch to use). I don't know; I'm up for elegant solutions.
A good chunk of the credit for this idea (which I think is a brilliant solution to the thematic inconsistency between Mirror Magic and its functionality near the Spire, personally--but your mileage may vary!) goes to a friend of mine who inspired it when he mentioned how mirror-magic is probably some sort of funky 'reversed' magic (thus it should work at the Spire just fine).