I have created some obscenely powerful spells before, but this one takes the cake. Because it is so powerful, I have several optional methods of control for it.
Name: Polymorph Any Spell
Level: 9th
Duration: permanent
Range: 60'
The caster can transform any spell of 8th or lower into any other spell of equal or lower level. It can be used in two different ways. The first is against on going effects. If it is cast upon an unwilling target, then that target receives a save versus spells to negate the transformation. If cast upon an unattended spell effect, the transformation is automatic. The second use of Polymorph Any Spell allows the caster to transform incoming spells that arrive the round PAS is cast or the round after. Both casters must make an Intelligence check and the higher result wins.
Now that is a pretty damn powerful spell even with the save and Intelligence check limitations. A Flesh to Stone spell could be turned into a Teleport spell, thus reverting the statue to normal and allowing the caster and victim to escape. Or it could be turned into Geas, thus saving the victim but turning them into a slave at the same time (if they fail the save versus Geas). Here are some suggestions on how to control its use:
Make it a 10th level spell. This could be a psionic enchantment (Dark Sun), quest (2e cleric ubermagic), epic (3.X), incantation (3.X) or something similar that is normally beyond the bounds of the PCs, but still attainable. If there are levels above 10th in your setting, then the level of spell that can be affected is 2 levels below the level of Polymorph Any Spell (not 1 as in the version above).
Make it a subset of Wish. If there are any negative consequences of Wish, this is the place to apply them.
Give it an extremely expensive material component or have a side effect/backlash. The rules for incantations in the 3.5 Unearthed Arcana has several very good ones but I am sure you can come up with more. For a material component, it shouldn't be just expensive, it should be rare. Like the feather of the roc that lives in the crown of the world tree. Just finding the bird, much less obtaining some of its feathers, is a major undertaking.
Break it up into specific spells, X to Y, each that has to be found or researched. So Flesh to Stone to Teleport is a different spell from Shield to Burning Hands.
Making it a spell that is restricted to a specific class. This could be a prestige class or (in LL/BD&D) a racial class. I suggest that you include one of the other limitations along with this one.
Make it a power of a relic or (major) artifact. With all the side effects and other negative consequences of these items, it may not even be worth having for very long.
