Okay, so that puts Storm of Vengeance into the Epic range. Not what you were looking for...
Okay, next...
Ice Storm, Sudden Widen, Sacred Spell, and Energy Substitution to Acid or Lightning, so it manifests as something that Undead don't laugh it. (Most are immune to Cold damage.) That makes it manifest as a 6th level spell.
The down side is that you can only use the Sudden effect once per day, and an 80 foot cylinder may seem pretty big on the battlemat, against a besieging army it's a pinprick, size wise. You'd need to do it more than once a day to get the epic scale you're looking for.
You could simply Widen the Ice Storm normally, which makes it take a 7th level slot, but making it Sacred would bump it to 9th. If you have multiple 9th level spells per day at your disposal, then this is a different scale of battle, and realistically the town is lost. The PCs may prevail, but anything that can challenge the party at that level will simply slaughter the rest of the population.
But let's be honest, any DM that lets you one-punch an entire army of undead isn't doing their job.
