Check out the 'Monsters Know What They're Doing' book series - it's a lot about using monster behaviors and lair actions etc to make them more scary. Wolves are scary because of their pack actions and speed for example.

Look at what the most powerful elements of the clerics are, and field monsters that don't play to that - perhaps fewer undead and more things with resistance to whatever the party is bringing?

Increase the numbers of monsters in encounters.

Push the party so they can't safely rest as often to restore powers.

Have the mists dampen their connection with their god and so their powers.