Way back in 2013 or so, before my old laptop was stolen and I lost about half of my projects, I was trying to make a Fire Emblem equivalent to Kaizo Mario, and a few chapters in there was a chapter called The Factory; the idea being that bandits are such a generic enemy, they were literally being churned out by a factory. At the top of the map, there were several of those cave/doorway tiles used in the ruins tileset, and every generic bandit boss from FE7 was dotted about here and there. They were the only enemy units, but every time one was defeated, they would immediately re-load out of one of the doorways and run back to their original location. This made for some "fun" situations where one of your units would kill a boss in retaliation and then the boss would immediately re-load and happen to end up next to another, weaker one of your units. The only way to win was to seize a throne, which was imagined to be the switch that turns off the factory, but if you just weakened the bosses instead of defeating them they would move to the throne to recover their HP. I hope to re-visit this concept someday in the form of a druid who has a shrine that empowers him to summon phantoms and the only way to stop the phantoms pouring out is to destroy the shrine.
edit: fwiw this was before Phoenix Mode existed; if Fates occupied a permanent position in my memory banks I wouldn't've posted this