My friend proposed an intriguing campaign concept: from a manga his reading called (Realist Demon King) where the players take on the roles of new Demon Lords in a world teeming with others like them. The objective? To establish their Demon empire as the most formidable force in the land. However, I’m grappling with how to execute this with multiple players without resorting to one player being the Demon Lord and the rest as their generals—especially since my players prefer more autonomy.

One alternative I’ve considered is forging alliances of non-aggression among the players, only to pit them against each other in a climactic showdown. But I’m unsure how to make this dynamic work seamlessly, as it seems odd for a Demon Lord not to travel with their minions.

I’m open to any suggestions on how to refine this concept into a cohesive and engaging campaign. Your insights would be greatly appreciated!

If you plan on doing PvP with your players, make sure to clear it with your players in session 0. Honestly, your idea works; nothing says the players can't rule as a consulate, similar to what the Romans did. Don't pit your players against each other unless they are well aware that it is going to happen.

As far as minions, the answer is that your group is a ragtag group of weaker demons who are too weak to rule on their own; all of the minions flock to more powerful demons. Throughout the campaign, the group could gain followers, but most of them are left behind to manage areas already conquered; after all, the players can't be everywhere at once, and someone has to manage conquered territory.

This concept could work well, but you will need to do a session zero to establish rules; make a list of things that must always remain true, and a list of ways to justify those in world. An important part of this is that unless it's agreed otherwise, the players and DM should try to keep these rules in place, even if it doesn't fully make sense in the moment. Good luck with the campaign, I hope you have fun with it! There's a lot to do with this concept.

Take the various optional Tiefling variants.   Cross out "Tiefling".  Write "Demon Lord" on their character sheet.

Have a Demon Queen give them specific rules about how they can interact with each other, preventing direct player vs player combat.  

Let them role play everything else.

Quote from KthunkTheBarbarian >>

If you plan on doing PvP with your players, make sure to clear it with your players in session 0. Honestly, your idea works; nothing says the players can't rule as a consulate, similar to what the Romans did. Don't pit your players against each other unless they are well aware that it is going to happen.

As far as minions, the answer is that your group is a ragtag group of weaker demons who are too weak to rule on their own; all of the minions flock to more powerful demons. Throughout the campaign, the group could gain followers, but most of them are left behind to manage areas already conquered; after all, the players can't be everywhere at once, and someone has to manage conquered territory.

This concept could work well, but you will need to do a session zero to establish rules; make a list of things that must always remain true, and a list of ways to justify those in world. An important part of this is that unless it's agreed otherwise, the players and DM should try to keep these rules in place, even if it doesn't fully make sense in the moment. Good luck with the campaign, I hope you have fun with it! There's a lot to do with this concept.

 

I think this is best possible way to work the concept without making any modifications to the existing 5e rules. You could play it for comedy, too; in a world full of demons all trying to dominate one another, your party (and only your party) has discovered a terrifying new power called: Mutually Beneficial Cooperation!

Now, if you did want to make modifications to the existing 5e rules, MCDM Productions has a supplement called Kingdoms and Warfare that you might find useful. I'm not saying you've got to run out and spend $30 to make the concept work, but it's out there if you want it.

I think you could do a lot of it through re-skinning. The tiefling idea above is good. Or really take any race mechanics-wise, and make cosmetic changes to look however you like. 
Instead of one of them being a demon lord, have them work for one, as an above poster said. You really don’t one character to be in a position of power. It creates a main character in a game that doesn’t do well with main characters. And what happens if the character dies or the player leaves the game? Keep the patron an NPC.

Finally, I’d suggest devils instead of demons. They have a better temperament to work together toward a common goal. But, of course, it’s your world, you can always make lore changes to make the concept fit. 

You could also grab some low-ish level fiend statblocks for them to choose from and use the Tasha's Cauldron sidekick rules to let them level. Keeping pressure on them with outside threats might also help ensure they don't scheme too much against each other too early. Possibly one of the subtler demon lords like Graz'zt or Lolth is using them as proxies, either openly or by posing as a "helpful advisor," and that might provide some soft direction until they know more about their patron's goals and have the power to turn against them.

i do a thing with my brother called DNDmon battles. we choose 2 monsters each and battle them. we ignore movement, traits, resistances, vulnerablilties, and saving throw proficencies. we always choose deamon or devil lords becaue their stupid powerful and battle until both the other player's things are dead. this is legit a cool idea and i'm gonna steal it shamelessly

You need the plot to be interesting enough to keep playing. Expanding their empire is nice, but they need a common enemy or difficult obstacle rather than going town to town conquering. Perhaps there is another demon far more powerful than the players.