Frankly it does not make sense tactically for Brevoy to open another front in the middle of war between it's two halves. Adding unnecessary enemies is one of the primary reasons Germany lost WWII so badly - though that actually proves that sometimes tactical mistakes are made (which goes towards supporting your idea).
However, a civil war has got to add an even larger complication. Not only are you at war, you can't even be certain of the loyalty of the majority of your people. Brevoy seems (to me)Game of Thrones-esque where every party is out to further their own agendas and they can/will do anything to reach their goals. If I was leading Brevoy's southern half I would be extremely worried about starting a war that will require forces be deployed south instead of north.
Now I realize that's not even close to answers and ideas like you wanted so I do apologize for that. I'm sure your players will enjoy the fighting though :)
Your players' armies are formidable to be sure. I doubt that Brevoy has armies of bound elementals, mage killers, or undead so overwhelming numerical superiority (led by powerful Swordlords whose martial mastery is almost beyond belief) is key.
Too bad undead armies all too often seem to reinforce themselves ;)
All Brevoy needs to do is send one of their NE bishops (clerics) into the offending border city in the dead of night armed with a bit of death magic and necromancy.
Kill some family of commoners, create greater undead a pair of wraiths and leave via word of recall. If you look at wraiths' spawn ability, within mere hours thousands of them will decimate the city by sunrise. Come sunset, the rest of the city dies. On the third day's sunset wraith armies will silently march across their nation eager to lay waste to all that they see...
^__^
All Brevoy needs to do is send one of their NE bishops (clerics) into the offending border city in the dead of night armed with a bit of death magic and necromancy.
Kill some family of commoners, create greater undead a pair of wraiths and leave via word of recall. If you look at wraiths' spawn ability, within mere hours thousands of them will decimate the city by sunrise. Come sunset, the rest of the city dies. On the third day's sunset wraith armies will silently march across their nation eager to lay waste to all that they see...
^__^
My party already plans on doing that.
The reason That there is a war with Brevoy is Varnhold. The party traded the souls of the entire community to the Guardian Daemon that was helping defend the lich's lair. Pharasma sent some accountants to settle the score and while the party was able to prevent civil war in their own country, Brevoy would not turn a blind eye toward such evil.Southern Brevoy controls the entire region around Varnhold. The PCs have made several threats toward Their northern neighbor stating that they would like to take Restov. and while peacefuuly would be preferred, war is an option. They decided they wanted a war on 2 fronts. so Pitax will attack the west, Brevoy the north
Your PLAYERS are planning to unleash wraithly armageddon upon the world? How appropo if they found the tactic being used on both/all three sides of the war .... :D
Perhaps this is a longshot, but it might work out fairly interestingly. Imagine that Brevoy catches wind of the player's inclinations, and the civil war to the north is set aside for a short time. Abeted if you will for the need to stop the growing, seething darkness to the south?
Maybe the players are taken by suprise as the *Might of the North* rides south, trampling these puny undead beneath the righteous crusade of the Brevan People's Front (Formerly known as the People's Front of Brevoy), in their effort to thwart these upstart rulers looking to plunder the wealth of Issia and Rostland?
Dunno, seems like it could be an interesting twist...