Mimic city; every building is a mimic, but they’re smart enough to eat the local population when they’re alone or in small groups. Every time people keep going missing, and the players will assume it must be some random wandering monster; they’d never think it’s literally the buildings and each room can start eating really fast. There’d be no pattern either, every building is alive and people go missing literally everywhere. Perhaps a clue can be a house that has no more people living in it and it starts to starve to death and starts freaking out, even endangering the other mimics’ secret.
I’ll do you one worse and I’ve posted this before: when you make a post looking for help on an esoteric problem, and then solve it and just reply ‘nvm fixed it’, and then years later have the same problem but when you go looking for the solution only find your old posts.
Give 'em hobgoblins; if I recall correctly, those guys have like 18 ac and teamwork abilities. They’re also really low CR; my brother’s party faced a group of them and could barely hit them. Cheap for sure, but a guaranteed difficult encounter.
Mimic city; every building is a mimic, but they’re smart enough to eat the local population when they’re alone or in small groups. Every time people keep going missing, and the players will assume it must be some random wandering monster; they’d never think it’s literally the buildings and each room can start eating really fast. There’d be no pattern either, every building is alive and people go missing literally everywhere. Perhaps a clue can be a house that has no more people living in it and it starts to starve to death and starts freaking out, even endangering the other mimics’ secret.