Almost 3yo scared of monsters

Hi everyone :)

So I have a daughter who will be turning 3 next month in June. I have 2 sister in laws, my husband sister, and his brothers wife. Whenever they don’t like something my daughter is doing (misbehaving, putting random things in her mouth, playing with something that might break, normal toddler behavior) they threaten her and say a monster will come and get her instead of stopping her like a normal person or redirecting her.

She’s never been exposed to any monster related thing ever, nor have I ever even said the word monster to her, but the fear is so bad she’s refusing to sleep in her room by herself, and even refusing to walk past a room that the lights are off because she’s scared there’s a monster in there.

I don’t think she even really knows what a monster is but the context that it’s used in just has scared her so much. I tried explaining to her that monsters are not real but I don’t think she’s even grasping what the concept of real/fake/imaginary is, yes she does a lot of pretend play but the actual words and concept obviously she can’t grasp yet.

Does anyone have any advice on how to solve this monster issue?