Well, you’re in a pickle. Characters are built up. Your party is level 10, and you need to avoid an arms race or probable TPKs that still challenge the players.
I would slow/stop advancement. Advance through flexibility. Not Have things that are much more powerful.
Have an army of some terrible power take control of their region, but the players keep their town protected. But it is only a matter of time.
Create a boss that is a herald of the new regional leader. A boss that has the damage output to kill any of the PCs in 2 rounds. And the ability to take their top damage for two rounds. Add some normal minions for it. And this guy is just 1 of 20+ who will be managing his new slaves.
Your PCs beat him. Probably in 4~5 rounds. But the conclusion is important.
- They know there is a new baddie in town.
- They know the might of this baddie is over their level.
- They do not know how the baddie is so powerful
- Their people are in danger.
- Tough fight and minimal reward?
Do they lead a rebellion?
Do the escape with their people?
Do they fortify their position?
Do they try to fight through the invaders, and kill the Big Bad Leader?
Do they just run, and leave their people to their fate?
Leave the players with options, new characters to act as agents of their main characters?
Play on as themselves?
Play as lords? With 3 agents each, all starting characters? Occasionally recruiting more.
Let the players determine the future, but if they continue on, advancement is slow, and the potential for death increases. Traps and ambushes get more common as they are more well known.
A great force is arrayed against them. Will they charge into certain doom, will they protect their people?
Will you start a Warpshell game?
This is the most basic way I would deal with your situation, for the future, I’d be slower with advancement.
This will give you at least 5 sessions to get your ducks in a row, as they attack other similar lieutenants, before the number of lieutenants and target numbers start to creep up and you eventually kill a couple of characters.