Usually when I see it it's a 500+ HP army, but I think this guy is strong enough to wipe out my city that he's about to attack. There are no cleared wildlands in this game, AFAIK.
http://darkseam.com/fe/When_Wildlands_Attack.EleSav
Okay, so he doesn't attack the city every turn. When you load the game, it should center on my sovereign. The city closest to him has a Naja Army inside its borders.
Another fun bug: Cast Tremor on the Naja's Army. Mouse over one of them and it says it'll last for 3 season. End the turn. Now check the duration. It will now say 0 seasons. Also, they will be able to move again when you end the turn. Either this or the game will lock up after End Turn. I've had both results.
Update (yeah, I know it's only been a few minutes): I tried using Tremor to keep my city alive so my Sov could get over to defend, but it was a moot point. A Troll Shaman army, one of those 500+ HP ones, showed up a few turns after my other posted save. Here's a savegame with both of them in my city borders. I don't know if I can fend off the Troll Shaman army. I guess I'm going to find out.
http://darkseam.com/fe/When_Wildlands_Attack_2.EleSav