I founded several cities in the thin strip of lush forest and green grassland allotted to me, wedged between the rough seas on one side and wasted brown hills on the other, when I received a quest to clear said wasteland of enemies. It's mostly a cakewalk. Find cave, clear it out with my carefully-constructed squad of elite random units, heal up, move on to the next cave. It's all going swimmingly, up until the last cave.
See, I've got a pretty good collection of units. Three heroes, four more of the best units I can currently construct. We've each leveled several times and the entire collection is labeled "dangerous". I'm pretty much able to rofflestomp anything and everything with minimal casualties (and what casualties there are is usually limited to my mages when autoresolving - why autoresolve puts the squishies on the front line, I have no clue). This last cave, however... The enemy has the initiative. One of the creatures bum rushes one of my heroes and takes him out with one attack. Then the rest of the enemy gets their turns. I can maybe take out one before getting wiped out.
Results? The previously unsettleable area can now be settled. My entire squad has been destroyed (though heroes can't be killed, just maimed). Oh, and there's now a superpowered enemy squad roaming the landscape that gains more experience if I attack it and lose. If there are heroes in the losing squad, they get maimed yet again, becoming less and less useful.
Fallen Enchantress.