This pc’s find themselves in yet another small town during their travels to location x, and as luck would have it,(good or bad depending on ones view) this town has a problem, a recent accumulation of ghosts have been plaguing the crypt and graveyard outside of town.
While specters haunting graveyards may not be overtly unusual, these spirits seem to be far more destructive. laying waste to entombed people and scattering their remains as well as their entrapments, across a wide area.
Only seen during the clearest nights when the moon is fullest, these ghosts have been known to speak in a gibbering almost entrancing voice and drive many people to the edge of sanity.
Naturally the townsfolk are appalled at the continual desecration of their deceased loved ones, and have tried several attempts to speak with and even exorcise these dreaded specters
So far any attempts to converse with or threaten the creatures have met with dismal failure, many of the townsfolk brave enough to confront them have been injured by a painful soul wrenching coldness, and some have even lost their mind entirely for a short time.
These "ghosts" are actually nothing more than a small group of thieving wizards, who cover their bodies with a fungus slime that glows eerily under the light of the full moon and use various magics to repel unwanted "exorcists."
This small group of grave robbers travels form town to town, appearing without warning and usually ransacking the local grave yard for a month or more. (depending on the amount of resealable goods they find on the dead) Once the graveyard has been picked clean they quietly and quickly move on to another town and repeat the process, usually stopping in town during the day and selling off their previously acquired goods form the last heist.
Many of the magics they use are illusion based, as well as mentally influencing, such as those that cause fear, or even charm a person for a time.
If a person proves too tenacious they will resort to some more physical spells, usually keeping them to cold based magics that have an area effect, so as to preserve the spiritual aspect of the encounter.
So far they have found much amusement in confronting clerics and priests that dare to face them with their holy spells, enjoying the infamy that comes with being far too powerful for even servants of the light to exorcise or harm.
Expanding the scenario
Perhaps in the tomb robbers wake the restless dead rise in these ransacked graveyards, and spread out to the nearby village searching for the culprits, causing no small spread of panic. (and pc’s who defeated the false ghosts may assume these zombies to be more of the same, and get a painful surprise.)
Perhaps this group is only a smaller branch of a larger band of magic wielding brigands, who were undergoing a initiation ritual before being allowed with in the ranks. if the pc’s find adequate clues to a more developed society of magical grave robbers it could lead to several adventures tracking them down and putting an end to their mischief.