I'm also not totally clear if you mean "forsaken" to be farther from the diving than "unholy," vice versa, or, if the two aren't correlated. Here is another way to think about it that could be fun:
The Extended Sanctity Continuum:
Very High Sanctity <> High Sanctity <> Normal Sanctity <> Low Sanctity <> Forsaken <> Low Unholy Sanctity <> Normal Unholy Sanctity <> High Unholy Sanctity <> Very High Unholy Sanctity
In this conceptualization, everything to the left is exactly as described in
Spells (5). "Forsaken" is the equivalent to "No Sanctity," meaning that it is fully devoid of all divine energy and clerical magic won't work. The right hand side is a mirror image of the left, and the levels of divine energy correspond, so the farther from the center you go (in either direction) the higher the level of divinity and the easier casting clerical magic becomes. . .
The catch ('cause what's the fun if there's no catch?) is that although there is divine energy to draw on in areas of unholy sanctity, it's the
wrong sort of divinity, and it is eager to corrupt our noble Cleric from the (presumably "good") gods they follow. Naturally, the higher the unholy sanctity, the more powerful its corrupting influence is, so while you can probably get away with drawing on Low Unholy Sanctity while resisting the temptation(s) of these unholy gods, a single spell cast in a place of Very High Unholy Sanctity is likely to fully corrupt the cleric to the whims of evil.
Probably a Will check would make the most sense here: Each time you cast a spell in an unholy space, roll versus will, penalized for the level of unholiness (say: -2 for low, -4 for normal, -8 for high, -16 for very high [
Yay! anoter reason to bump your will to 25!]), any failure leads to corruption.
Now, what that corruption actually looks like, I have no idea. That's something for me to pray to the Elders over. . .