I wonder how many "good" characters might eventually fall to evil (or at least non-Good) after seeing success after success result in little or no change. The desire to cause some change - a true and lasting change - might grow so great that the characters might fall into a "the ends justify the means" mindset - with the destruction, banishment, or at least reduction of the power of the Shadow and its minions being the "ends" rather than the freedom / aid of those subjugated (which might have been the original "ends" of the party). It could end up being an interesting group with an interesting future. They might succeed - only to realize after the fact that their success, their first great and lasting success, has caused more turmoil and trouble than the Shadow had caused . . . .
What if they succeed in somehow reducing the Shadow's power greatly, slay its major priests, champions, etc, create discord amongst its minions so that they fall amongst each other - decimating themselves, and finally manage to seal or otherwise contain (or at least reduce in power to such an extent that it is no longer as menacing or able to effect all that much) the Shadow. And then all those that died rise as hordes of undead: zombies, ghouls, wraiths, shadows, etc. The deities are still beyond reach, so Turning is arguably not available, and now the word is facing something akin to Dawn of the Dead or some such. Or perhaps the people do not return as zombies or wraiths but instead are still trapped ghosts. Suddenly most of the population on the world are dwarves, elves, or ghosts (mostly of humans or orcs, I imagine).
It could be interesting, I think . . . .