There's no reason the conquerors have to be the same species. There's a lot of precedent in all known life, parasites, symbiosis and predator/prey relationships

There's no reason the conquerors have to be the same species. There's a lot of precedent in all known life, parasites, symbiosis and predator/prey relationships are everywhere. Nor does it have to be a personified antagonist, could be climate change, plague, etc. The longer the system goes the more shocks it has to survive.