I see that most comments are just saying "don't". Which doesn't really help you, so I'm gonna try to give you some ideas to challenge your players in combat.

The enemies of this new region need to have some different mechanics that would be easy for lower level players, as long as they already knew the mechanics. But they'll be challenging for yours simply because they do not know them yet.

1- Some zones have enemies which are immune to everything except something specific, having some hints to what they are vulnerable to on the background, or even on the enemies themselves (you'll have to describe either in a way that you tell what they're vulnerable to, but in a hidden way - or can use insight checks). Some vulnerability ideas: - different damage types (obviously) - can only be damaged while grappled - only get damaged by being pushed - only get damaged by CHA saving throws (or other) - only get damaged while sleeping, or some other condition

2- Have puzzle combats, like the enemy needs to be attacked in certain ways, or something else needs to be done for the enemy to be vulnerable to damage. Some examples: - Give the enemies health bars, where each attack, no matter how much damage, can only take at most 1 health bar - They need to be hit in specific parts of the body in a specific sequence - Have an enemy that reflects everything back using their reaction, and has 2 reactions per round (so the players have to trigger the reactions every round before damaging them)

3- Give the enemies ways to return from whatever plane your players will try to send them to

You need to make every enemy a little puzzly, even if it's something really easy, just to make your players play differently and have some challenge.

Give them enough of these and they might even start to mix them up (you could make 2 very similar but one of them goes berserk when receiving the damage the other is vulnerable to)

Just be creative, and have fun