[Alneyan]

Crumble doesn't do a lot of damage though, and will probably not work if he really has *that* many more undead. If you can start breaking down his gates, then Crumble is a very good option; if not, it will probably not work, as the defenders get the opportunity to repair their walls after the spell has been cast.

There is no other spell to help breaking down the forts themselves, and no event will destroy a fort. The good thing is, the two spells increasing fort defence do not work, so you won't have to worry about that.

I can't say I have any specific advice, though, beyond Flames from the Skies and similar spells (the more, the merrier). Additionally, you might wish to play a bit with Mind Hunt: even Immortal commanders can be annoyed with this spell, and it may disrupt his plans. If he used Stygian Paths, he probably doesn't have easy access to Astral magic, so you might be able to use this spell to good effect.