Felix said:
It's going to take a few minutes for my eyes to roll into place, so far back did they go after reading this.
Did someone try to use this, or was it just a let's-see-if-we-can-break-the-game exercise? I hope it was simply a thought experiment...
Found the build...
Illumian (Naenhoon) Cleric 6
Feats: Heighten Spell (1), Sanctum Spell (3), Extra Slot (6)
Thats it.
So whats the trick?
That 6th level feat is Extra Spell for a 9th level spell slot
How?
Here goes:
The Illumian Runeword Naenhoon from Races of Destiny gives the character a limited version of divine metamagic, whereby they may spend turning attempts to apply metamagic. Specifically with heighten spell the illumian may spend one turning attempt per level to be heightened, but cannot increase past 9th level.
Sanctum Spell allows us to cast a spell without adjusting its slot level. If this spell is cast in a designated sanctum it functions at one level higher than normal, if cast beyond that, it functions one level lower.
So when we get to taking that level 6 feat, we select extra slot, which tells us that we may choose any spell level up to one less than the highest level spell we can cast.
What is the hightest level spell we can cast. Well if we cast in our sanctum, and use our runeword, we can actually cast a 10th level spell. Now, its actually a 3rd level spell, but it counts as a 10th level spell (save DC is 20+casting mod and everything). By the wording of extra slot, we can select an extra 9th level spell slot, taking us to a grand total of 1 in that regard.
Now if we have 19 wisdom, we can prepare spells like miracle, gate, implosion, mass heal, energy drain. You know, the fun stuff.