The overland map may contain multiple enemy forces, including lurking bands occupying hostile sites, more aggressive raiders infesting both civilized and wild lands, and open armies on the march making incursions.
Every known enemy force on the map is shown as a small coin on the map and also as a square monster portrait at the top right of the screen. (The coin may be hard to notice because it's often drawn overlapping with the heroes.) Scouting wild lands may reveal new enemy forces.
Clicking on either the coin or the portrait will bring up an assault job that can be used to initiate battle with that force.
Every enemy force has a strength, which determines the number of foe cards it draws at the start of a battle. Strength starts at a base of 4 for incursion armies and 3 for other forces. It is increased by +1 for each territory the force has infested.
Monsters are infesting this tile. They may ambush groups of three or fewer heroes. Patrol or take out the source to clear the infestation.
Every infestation has an origin force. The origin force may be an incursion army, or it may be a lurking force present at the beginning of the chapter. If the infestation has spread from an unexplored wild land, the force will not be revealed until you have scouted its origin territory. You can identify which infested territories belong to a given enemy force by hovering over either the force coin or the monster portrait, highlighting their territory.
Each territory infested by an enemy force adds +1 to its strength.
If infestations are not cleared away, they will spread to adjacent previously-safe tiles.
Infestations can be cleared either by patrolling or by assaulting the origin force. Patrolling first is not necessary, but it makes the battle easier.
Clearing away infestations earns better outcomes at the conclusion of a chapter.
An incursion begins by spawning a monster army in a wilderness tile. They immediately begin moving into a developed territory -- this is shown as a red dotted line on the overland map. Hovering over the destination territory shows how soon the army will arrive. If desired, the monster army can be attacked in its origin territory right away.
When the army arrives, any heroes in the invaded territory must immediately fight a Defense battle. If they lose, retreat, or weren't there to fight off the army at all, the territory is conquered and converts from civilized back to wilderness. The monster army remains, and soon will begin moving again to invade another adjacent territory.
If the heroes fail to defend a town (lose, retreat, or are not able to fight), it will be destroyed upon occupation and can't be rebuilt.
Heroes can gain various advantages in the defense battle by performing the Prepare Defense job in the territory before the invading army arrives. Up to four different benefits can be gained by completing the job four times.