Spook!

This mod adds a single mob, the Spook. They are hostile and will attack the player with their melee dash attack or ranged projectile attack if not within melee range. When attacked they will disappear then reappear in a nearby location, they often catch the player off guard above or behind them. The Spook are nearly harmless in the light and will fear players, backing off to a safe distance or teleporting away if backed into a corner. They will search for torches to break if the player is not near to distract them. They won't survive long out in the open, even in the nighttime they will quickly despawn. You can find them summoning from defeated undead mobs in the overworld or wandering the nether wastes and soul sand valleys.

This mod is no longer being supported, instead check out the new version of the Spook in lummobs! - https://www.curseforge.com/minecraft/mc-mods/lummobs