Above your minimap, you will see a Sun or a Moon, indicating whether it is daytime or nighttime within Barovia.
While it is nighttime, there is a chance that a miniboss creature called the Night Terror will spawn randomly around the map. When you are near it, you will see a healthbar pop up at the top middle of your screen with the name Night Terror. You need to do a reasonable amount of damage to the Night Terror to get credit towards defeating it. This miniboss is only available at night.