Hey Juust! Dude, that sounds like an awesome horror game idea! Ok, I'm not the most experienced at this but I can try to offer what I would do?
Alright, so I would add a condition to the monster to check if it's collided with the player. Then add a sound effect and maybe a scary image as an action to make the jumpscare effect.
This does seem a bit like a noob solution to this, so I think it would be a good idea to try to get a second opinion.
Thanks! And good luck!