We had a bit of a discussion about this a long time ago, starting here:
https://forums.civfanatics.com/threads/ideas-requests-and-feedback.373631/post-11061620
@Krieger66 suggested a new world spell: "The Forsaken Path: For 5 turns infernal units that die in combat create a hellish flames in the square they died, that spawns units for the infernal civ.(The AI would be weighted to use this during a war, or when they have a stack of units in enemy territory). Enemy units killed add mana to your stockpile equal to (.5 * level), or some other value depending on how mana is handled."
Then
@Valkrionn suggested the following: "I would instead say that all existing units receive a promotion which causes them to spawn this fire when they die, and then limit how long the fire itself lasts, rather than limit how long they can be created."
I know I'm quoting a discussion from 14(!) years ago, but I'm afraid my own suggestions for a new Infernal World Spell weren't too creative.
If the current world spell stays, maybe it should function as some kind of a balancing mechanism, targeting the civ with the highest score. Although losing 3 or 4 or 5 cities would suck, even if you're the most powerful civ in the world. Yes, you can quickly take back the cities in question, but you'll have to rebuild several buildings. And if you've lost a city with one or more National Wonders it's even worse.