Jumpscare Help?

I'm making a jumpscare for a horror mod. I want it to happen when the entity is watching the player (only during the day) and not too often, as I don't want it to get repetitive. The jumpscare is an image overlay and a sound, but it only lasts for a few seconds, and when the image disappears, the player has blindness for about 3 seconds, and the player gets an achievement. Help?