Watchmen

These mobs observe you from a distance and will only spawn/attack during night or any time of day in caves but burn if caught out during the day. The only way to prevent an attack is to stare at them until you kill them or until daylight arises. I would imagine they resemble a ghost like floating mob that remains at ground level at all times (or be a variant of the Vexes and be larger while not floating as high) and do 2-3 hearts of damage but only up close, no ranged attacks, also dropping slime balls since they are made of a solid form of plasma or ectoplasm which can make them related to the Slime leading to you seeing them occasionally attacking each other for dominance.