Nebulous
Legend
But how could you circumvent the banishment thing if you wanted to? One solution would be to have the balor already been in action and gating in some more devils, I think he could do that. Then his support troops would agro the most helpless population. Everyone not first stopping them (waisting the banishment divinations) is in for an alignment change.
I would go a step further. If your big baddie is extraplanar and you KNOW the PCs are going to try to banish it and end the fight, I would give it some hokey magic item that absorbs/deflects/redirects the banishment spell and only the banishment spell. The wizard getting banished by his own stupid spell would be pretty sweet
