So simply put, you have entered a new game. And that is perfectly fine if you are willing to run it.
So what would happen? Ok since the PC forces have been "so terrifying" and did so much damage, the city is probably going to reach out to help. Maybe they call out to a big city in the area, or maybe even the king's palace.
And to answer the call, a King's enforcer, or a cabal of wizards, or another adventuring group. These have the approval of law and the resources of the kingdom at their disposal. They use tracking and scrying to hunt down the pcs, and they don't' relent. If the party rests, they are harried. This continues until either the party leaves the kingdom, forever being branded as criminals in that kingdom (but now they can "start fresh" in a new place), they are killed, or they find a way to make a deal with someone to clear their names if they do something for the king, etc etc.
Have the hunters act like proper lawmen, and note that by Magistrate's edict, the criminals are to be "shot on sight". The PCs pictures go up on wanted posters, etc etc. If there is a paladin in the group, well most likely their powers are gone. If there's a cleric, they get a harsh talking to from their god and maybe lose their powers as well.
Its a new game, and it can be a fun game filled with interesting new roleplaying potential....but its not for everyone. If you don't want to run that, then its time to talk to the players about some kind of reset. But if you do, have fun with it, play it logically.... and think about this as a big magistrate dealing with a threat that just butchered a city's enforcers..... you can't handle that lightly, you need to send a powerful message and crush this force quickly. So you don't hold back, you would send a force that way over CRs the party, with the intention to destroy them utterly. The party has kicked over the beehive, now they suffer the consequences.
So what would happen? Ok since the PC forces have been "so terrifying" and did so much damage, the city is probably going to reach out to help. Maybe they call out to a big city in the area, or maybe even the king's palace.
And to answer the call, a King's enforcer, or a cabal of wizards, or another adventuring group. These have the approval of law and the resources of the kingdom at their disposal. They use tracking and scrying to hunt down the pcs, and they don't' relent. If the party rests, they are harried. This continues until either the party leaves the kingdom, forever being branded as criminals in that kingdom (but now they can "start fresh" in a new place), they are killed, or they find a way to make a deal with someone to clear their names if they do something for the king, etc etc.
Have the hunters act like proper lawmen, and note that by Magistrate's edict, the criminals are to be "shot on sight". The PCs pictures go up on wanted posters, etc etc. If there is a paladin in the group, well most likely their powers are gone. If there's a cleric, they get a harsh talking to from their god and maybe lose their powers as well.
Its a new game, and it can be a fun game filled with interesting new roleplaying potential....but its not for everyone. If you don't want to run that, then its time to talk to the players about some kind of reset. But if you do, have fun with it, play it logically.... and think about this as a big magistrate dealing with a threat that just butchered a city's enforcers..... you can't handle that lightly, you need to send a powerful message and crush this force quickly. So you don't hold back, you would send a force that way over CRs the party, with the intention to destroy them utterly. The party has kicked over the beehive, now they suffer the consequences.